diff options
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/i386/Kconfig | 10 | ||||
| -rw-r--r-- | arch/i386/Makefile | 3 | ||||
| -rw-r--r-- | arch/x86/lguest/Kconfig | 8 | ||||
| -rw-r--r-- | arch/x86/lguest/Makefile | 1 | ||||
| -rw-r--r-- | arch/x86/lguest/boot.c | 1106 | ||||
| -rw-r--r-- | arch/x86/lguest/i386_head.S | 93 |
6 files changed, 1213 insertions, 8 deletions
diff --git a/arch/i386/Kconfig b/arch/i386/Kconfig index 3523e82c841..5bed8be34ba 100644 --- a/arch/i386/Kconfig +++ b/arch/i386/Kconfig @@ -257,14 +257,8 @@ config VMI at the moment), by linking the kernel to a GPL-ed ROM module provided by the hypervisor. -config LGUEST_GUEST - bool "Lguest guest support" - select PARAVIRT - depends on !X86_PAE - help - Lguest is a tiny in-kernel hypervisor. Selecting this will - allow your kernel to boot under lguest. This option will increase - your kernel size by about 6k. If in doubt, say N. +source "arch/x86/lguest/Kconfig" + endif config ACPI_SRAT diff --git a/arch/i386/Makefile b/arch/i386/Makefile index b88e47ca303..b81cb64d48e 100644 --- a/arch/i386/Makefile +++ b/arch/i386/Makefile @@ -99,6 +99,9 @@ core-$(CONFIG_X86_ES7000) := arch/x86/mach-es7000/ # Xen paravirtualization support core-$(CONFIG_XEN) += arch/x86/xen/ +# lguest paravirtualization support +core-$(CONFIG_LGUEST_GUEST) += arch/x86/lguest/ + # default subarch .h files mflags-y += -Iinclude/asm-x86/mach-default diff --git a/arch/x86/lguest/Kconfig b/arch/x86/lguest/Kconfig new file mode 100644 index 00000000000..0fabf87db99 --- /dev/null +++ b/arch/x86/lguest/Kconfig @@ -0,0 +1,8 @@ +config LGUEST_GUEST + bool "Lguest guest support" + select PARAVIRT + depends on !X86_PAE + help + Lguest is a tiny in-kernel hypervisor. Selecting this will + allow your kernel to boot under lguest. This option will increase + your kernel size by about 6k. If in doubt, say N. diff --git a/arch/x86/lguest/Makefile b/arch/x86/lguest/Makefile new file mode 100644 index 00000000000..27f0c9ed7f6 --- /dev/null +++ b/arch/x86/lguest/Makefile @@ -0,0 +1 @@ +obj-y := i386_head.o boot.o diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c new file mode 100644 index 00000000000..8e9e485a5cf --- /dev/null +++ b/arch/x86/lguest/boot.c @@ -0,0 +1,1106 @@ +/*P:010 + * A hypervisor allows multiple Operating Systems to run on a single machine. + * To quote David Wheeler: "Any problem in computer science can be solved with + * another layer of indirection." + * + * We keep things simple in two ways. First, we start with a normal Linux + * kernel and insert a module (lg.ko) which allows us to run other Linux + * kernels the same way we'd run processes. We call the first kernel the Host, + * and the others the Guests. The program which sets up and configures Guests + * (such as the example in Documentation/lguest/lguest.c) is called the + * Launcher. + * + * Secondly, we only run specially modified Guests, not normal kernels. When + * you set CONFIG_LGUEST to 'y' or 'm', this automatically sets + * CONFIG_LGUEST_GUEST=y, which compiles this file into the kernel so it knows + * how to be a Guest. This means that you can use the same kernel you boot + * normally (ie. as a Host) as a Guest. + * + * These Guests know that they cannot do privileged operations, such as disable + * interrupts, and that they have to ask the Host to do such things explicitly. + * This file consists of all the replacements for such low-level native + * hardware operations: these special Guest versions call the Host. + * + * So how does the kernel know it's a Guest? The Guest starts at a special + * entry point marked with a magic string, which sets up a few things then + * calls here. We replace the native functions various "paravirt" structures + * with our Guest versions, then boot like normal. :*/ + +/* + * Copyright (C) 2006, Rusty Russell <rusty@rustcorp.com.au> IBM Corporation. + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 2 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, but + * WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or + * NON INFRINGEMENT. See the GNU General Public License for more + * details. + * + * You should have received a copy of the GNU General Public License + * along with this program; if not, write to the Free Software + * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. + */ +#include <linux/kernel.h> +#include <linux/start_kernel.h> +#include <linux/string.h> +#include <linux/console.h> +#include <linux/screen_info.h> +#include <linux/irq.h> +#include <linux/interrupt.h> +#include <linux/clocksource.h> +#include <linux/clockchips.h> +#include <linux/lguest.h> +#include <linux/lguest_launcher.h> +#include <linux/lguest_bus.h> +#include <asm/paravirt.h> +#include <asm/param.h> +#include <asm/page.h> +#include <asm/pgtable.h> +#include <asm/desc.h> +#include <asm/setup.h> +#include <asm/e820.h> +#include <asm/mce.h> +#include <asm/io.h> + +/*G:010 Welcome to the Guest! + * + * The Guest in our tale is a simple creature: identical to the Host but + * behaving in simplified but equivalent ways. In particular, the Guest is the + * same kernel as the Host (or at least, built from the same source code). :*/ + +/* Declarations for definitions in lguest_guest.S */ +extern char lguest_noirq_start[], lguest_noirq_end[]; +extern const char lgstart_cli[], lgend_cli[]; +extern const char lgstart_sti[], lgend_sti[]; +extern const char lgstart_popf[], lgend_popf[]; +extern const char lgstart_pushf[], lgend_pushf[]; +extern const char lgstart_iret[], lgend_iret[]; +extern void lguest_iret(void); + +struct lguest_data lguest_data = { + .hcall_status = { [0 ... LHCALL_RING_SIZE-1] = 0xFF }, + .noirq_start = (u32)lguest_noirq_start, + .noirq_end = (u32)lguest_noirq_end, + .blocked_interrupts = { 1 }, /* Block timer interrupts */ +}; +static cycle_t clock_base; + +/*G:035 Notice the lazy_hcall() above, rather than hcall(). This is our first + * real optimization trick! + * + * When lazy_mode is set, it means we're allowed to defer all hypercalls and do + * them as a batch when lazy_mode is eventually turned off. Because hypercalls + * are reasonably expensive, batching them up makes sense. For example, a + * large mmap might update dozens of page table entries: that code calls + * paravirt_enter_lazy_mmu(), does the dozen updates, then calls + * lguest_leave_lazy_mode(). + * + * So, when we're in lazy mode, we call async_hypercall() to store the call for + * future processing. When lazy mode is turned off we issue a hypercall to + * flush the stored calls. + */ +static void lguest_leave_lazy_mode(void) +{ + paravirt_leave_lazy(paravirt_get_lazy_mode()); + hcall(LHCALL_FLUSH_ASYNC, 0, 0, 0); +} + +static void lazy_hcall(unsigned long call, + unsigned long arg1, + unsigned long arg2, + unsigned long arg3) +{ + if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) + hcall(call, arg1, arg2, arg3); + else + async_hcall(call, arg1, arg2, arg3); +} + +/* async_hcall() is pretty simple: I'm quite proud of it really. We have a + * ring buffer of stored hypercalls which the Host will run though next time we + * do a normal hypercall. Each entry in the ring has 4 slots for the hypercall + * arguments, and a "hcall_status" word which is 0 if the call is ready to go, + * and 255 once the Host has finished with it. + * + * If we come around to a slot which hasn't been finished, then the table is + * full and we just make the hypercall directly. This has the nice side + * effect of causing the Host to run all the stored calls in the ring buffer + * which empties it for next time! */ +void async_hcall(unsigned long call, + unsigned long arg1, unsigned long arg2, unsigned long arg3) +{ + /* Note: This code assumes we're uniprocessor. */ + static unsigned int next_call; + unsigned long flags; + + /* Disable interrupts if not already disabled: we don't want an + * interrupt handler making a hypercall while we're already doing + * one! */ + local_irq_save(flags); + if (lguest_data.hcall_status[next_call] != 0xFF) { + /* Table full, so do normal hcall which will flush table. */ + hcall(call, arg1, arg2, arg3); + } else { + lguest_data.hcalls[next_call].eax = call; + lguest_data.hcalls[next_call].edx = arg1; + lguest_data.hcalls[next_call].ebx = arg2; + lguest_data.hcalls[next_call].ecx = arg3; + /* Arguments must all be written before we mark it to go */ + wmb(); + lguest_data.hcall_status[next_call] = 0; + if (++next_call == LHCALL_RING_SIZE) + next_call = 0; + } + local_irq_restore(flags); +} +/*:*/ + +/* Wrappers for the SEND_DMA and BIND_DMA hypercalls. This is mainly because + * Jeff Garzik complained that __pa() should never appear in drivers, and this + * helps remove most of them. But also, it wraps some ugliness. */ +void lguest_send_dma(unsigned long key, struct lguest_dma *dma) +{ + /* The hcall might not write this if something goes wrong */ + dma->used_len = 0; + hcall(LHCALL_SEND_DMA, key, __pa(dma), 0); +} + +int lguest_bind_dma(unsigned long key, struct lguest_dma *dmas, + unsigned int num, u8 irq) +{ + /* This is the only hypercall which actually wants 5 arguments, and we + * only support 4. Fortunately the interrupt number is always less + * than 256, so we can pack it with the number of dmas in the final + * argument. */ + if (!hcall(LHCALL_BIND_DMA, key, __pa(dmas), (num << 8) | irq)) + return -ENOMEM; + return 0; +} + +/* Unbinding is the same hypercall as binding, but with 0 num & irq. */ +void lguest_unbind_dma(unsigned long key, struct lguest_dma *dmas) +{ + hcall(LHCALL_BIND_DMA, key, __pa(dmas), 0); +} + +/* For guests, device memory can be used as normal memory, so we cast away the + * __iomem to quieten sparse. */ +void *lguest_map(unsigned long phys_addr, unsigned long pages) +{ + return (__force void *)ioremap(phys_addr, PAGE_SIZE*pages); +} + +void lguest_unmap(void *addr) +{ + iounmap((__force void __iomem *)addr); +} + +/*G:033 + * Here are our first native-instruction replacements: four functions for + * interrupt control. + * + * The simplest way of implementing these would be to have "turn interrupts + * off" and "turn interrupts on" hypercalls. Unfortunately, this is too slow: + * these are by far the most commonly called functions of those we override. + * + * So instead we keep an "irq_enabled" field inside our "struct lguest_data", + * which the Guest can update with a single instruction. The Host knows to + * check there when it wants to deliver an interrupt. + */ + +/* save_flags() is expected to return the processor state (ie. "eflags"). The + * eflags word contains all kind of stuff, but in practice Linux only cares + * about the interrupt flag. Our "save_flags()" just returns that. */ +static unsigned long save_fl(void) +{ + return lguest_data.irq_enabled; +} + +/* "restore_flags" just sets the flags back to the value given. */ +static void restore_fl(unsigned long flags) +{ + lguest_data.irq_enabled = flags; +} + +/* Interrupts go off... */ +static void irq_disable(void) +{ + lguest_data.irq_enabled = 0; +} + +/* Interrupts go on... */ +static void irq_enable(void) +{ + lguest_data.irq_enabled = X86_EFLAGS_IF; +} +/*:*/ +/*M:003 Note that we don't check for outstanding interrupts when we re-enable + * them (or when we unmask an interrupt). This seems to work for the moment, + * since interrupts are rare and we'll just get the interrupt on the next timer + * tick, but when we turn on CONFIG_NO_HZ, we should revisit this. One way + * would be to put the "irq_enabled" field in a page by itself, and have the + * Host write-protect it when an interrupt comes in when irqs are disabled. + * There will then be a page fault as soon as interrupts are re-enabled. :*/ + +/*G:034 + * The Interrupt Descriptor Table (IDT). + * + * The IDT tells the processor what to do when an interrupt comes in. Each + * entry in the table is a 64-bit descriptor: this holds the privilege level, + * address of the handler, and... well, who cares? The Guest just asks the + * Host to make the change anyway, because the Host controls the real IDT. + */ +static void lguest_write_idt_entry(struct desc_struct *dt, + int entrynum, u32 low, u32 high) +{ + /* Keep the local copy up to date. */ + write_dt_entry(dt, entrynum, low, high); + /* Tell Host about this new entry. */ + hcall(LHCALL_LOAD_IDT_ENTRY, entrynum, low, high); +} + +/* Changing to a different IDT is very rare: we keep the IDT up-to-date every + * time it is written, so we can simply loop through all entries and tell the + * Host about them. */ +static void lguest_load_idt(const struct Xgt_desc_struct *desc) +{ + unsigned int i; + struct desc_struct *idt = (void *)desc->address; + + for (i = 0; i < (desc->size+1)/8; i++) + hcall(LHCALL_LOAD_IDT_ENTRY, i, idt[i].a, idt[i].b); +} + +/* + * The Global Descriptor Table. + * + * The Intel architecture defines another table, called the Global Descriptor + * Table (GDT). You tell the CPU where it is (and its size) using the "lgdt" + * instruction, and then several other instructions refer to entries in the + * table. There are three entries which the Switcher needs, so the Host simply + * controls the entire thing and the Guest asks it to make changes using the + * LOAD_GDT hypercall. + * + * This is the opposite of the IDT code where we have a LOAD_IDT_ENTRY + * hypercall and use that repeatedly to load a new IDT. I don't think it + * really matters, but wouldn't it be nice if they were the same? + */ +static void lguest_load_gdt(const struct Xgt_desc_struct *desc) +{ + BUG_ON((desc->size+1)/8 != GDT_ENTRIES); + hcall(LHCALL_LOAD_GDT, __pa(desc->address), GDT_ENTRIES, 0); +} + +/* For a single GDT entry which changes, we do the lazy thing: alter our GDT, + * then tell the Host to reload the entire thing. This operation is so rare + * that this naive implementation is reasonable. */ +static void lguest_write_gdt_entry(struct desc_struct *dt, + int entrynum, u32 low, u32 high) +{ + write_dt_entry(dt, entrynum, low, high); + hcall(LHCALL_LOAD_GDT, __pa(dt), GDT_ENTRIES, 0); +} + +/* OK, I lied. There are three "thread local storage" GDT entries which change + * on every context switch (these three entries are how glibc implements + * __thread variables). So we have a hypercall specifically for this case. */ +static void lguest_load_tls(struct thread_struct *t, unsigned int cpu) +{ + /* There's one problem which normal hardware doesn't have: the Host + * can't handle us removing entries we're currently using. So we clear + * the GS register here: if it's needed it'll be reloaded anyway. */ + loadsegment(gs, 0); + lazy_hcall(LHCALL_LOAD_TLS, __pa(&t->tls_array), cpu, 0); +} + +/*G:038 That's enough excitement for now, back to ploughing through each of + * the different pv_ops structures (we're about 1/3 of the way through). + * + * This is the Local Descriptor Table, another weird Intel thingy. Linux only + * uses this for some strange applications like Wine. We don't do anything + * here, so they'll get an informative and friendly Segmentation Fault. */ +static void lguest_set_ldt(const void *addr, unsigned entries) +{ +} + +/* This loads a GDT entry into the "Task Register": that entry points to a + * structure called the Task State Segment. Some comments scattered though the + * kernel code indicate that this used for task switching in ages past, along + * with blood sacrifice and astrology. + * + * Now there's nothing interesting in here that we don't get told elsewhere. + * But the native version uses the "ltr" instruction, which makes the Host + * complain to the Guest about a Segmentation Fault and it'll oops. So we + * override the native version with a do-nothing version. */ +static void lguest_load_tr_desc(void) +{ +} + +/* The "cpuid" instruction is a way of querying both the CPU identity + * (manufacturer, model, etc) and its features. It was introduced before the + * Pentium in 1993 and keeps getting extended by both Intel and AMD. As you + * might imagine, after a decade and a half this treatment, it is now a giant + * ball of hair. Its entry in the current Intel manual runs to 28 pages. + * + * This instruction even it has its own Wikipedia entry. The Wikipedia entry + * has been translated into 4 languages. I am not making this up! + * + * We could get funky here and identify ourselves as "GenuineLguest", but + * instead we just use the real "cpuid" instruction. Then I pretty much turned + * off feature bits until the Guest booted. (Don't say that: you'll damage + * lguest sales!) Shut up, inner voice! (Hey, just pointing out that this is + * hardly future proof.) Noone's listening! They don't like you anyway, + * parenthetic weirdo! + * + * Replacing the cpuid so we can turn features off is great for the kernel, but + * anyone (including userspace) can just use the raw "cpuid" instruction and + * the Host won't even notice since it isn't privileged. So we try not to get + * too worked up about it. */ +static void lguest_cpuid(unsigned int *eax, unsigned int *ebx, + unsigned int *ecx, unsigned int *edx) +{ + int function = *eax; + + native_cpuid(eax, ebx, ecx, edx); + switch (function) { + case 1: /* Basic feature request. */ + /* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */ + *ecx &= 0x00002201; + /* SSE, SSE2, FXSR, MMX, CMOV, CMPXCHG8B, FPU. */ + *edx &= 0x07808101; + /* The Host can do a nice optimization if it knows that the + * kernel mappings (addresses above 0xC0000000 or whatever + * PAGE_OFFSET is set to) haven't changed. But Linux calls + * flush_tlb_user() for both user and kernel mappings unless + * the Page Global Enable (PGE) feature bit is set. */ + *edx |= 0x00002000; + break; + case 0x80000000: + /* Futureproof this a little: if they ask how much extended + * processor information there is, limit it to known fields. */ + if (*eax > 0x80000008) + *eax = 0x80000008; + break; + } +} + +/* Intel has four control registers, imaginatively named cr0, cr2, cr3 and cr4. + * I assume there's a cr1, but it hasn't bothered us yet, so we'll not bother + * it. The Host needs to know when the Guest wants to change them, so we have + * a whole series of functions like read_cr0() and write_cr0(). + * + * We start with CR0. CR0 allows you to turn on and off all kinds of basic + * features, but Linux only really cares about one: the horrifically-named Task + * Switched (TS) bit at bit 3 (ie. 8) + * + * What does the TS bit do? Well, it causes the CPU to trap (interrupt 7) if + * the floating point unit is used. Which allows us to restore FPU state + * lazily after a task switch, and Linux uses that gratefully, but wouldn't a + * name like "FPUTRAP bit" be a little less cryptic? + * + * We store cr0 (and cr3) locally, because the Host never changes it. The + * Guest sometimes wants to read it and we'd prefer not to bother the Host + * unnecessarily. */ +static unsigned long current_cr0, current_cr3; +static void lguest_write_cr0(unsigned long val) +{ + /* 8 == TS bit. */ + lazy_hcall(LHCALL_TS, val & 8, 0, 0); + current_cr0 = val; +} + +static unsigned long lguest_read_cr0(void) +{ + return current_cr0; +} + +/* Intel provided a special instruction to clear the TS bit for people too cool + * to use write_cr0() to do it. This "clts" instruction is faster, because all + * the vowels have been optimized out. */ +static void lguest_clts(void) +{ + lazy_hcall(LHCALL_TS, 0, 0, 0); + current_cr0 &= ~8U; +} + +/* CR2 is the virtual address of the last page fault, which the Guest only ever + * reads. The Host kindly writes this into our "struct lguest_data", so we + * just read it out of there. */ +static unsigned long lguest_read_cr2(void) +{ + return lguest_data.cr2; +} + +/* CR3 is the current toplevel pagetable page: the principle is the same as + * cr0. Keep a local copy, and tell the Host when it changes. */ +static void lguest_write_cr3(unsigned long cr3) +{ + lazy_hcall(LHCALL_NEW_PGTABLE, cr3, 0, 0); + current_cr3 = cr3; +} + +static unsigned long lguest_read_cr3(void) +{ + return current_cr3; +} + +/* CR4 is used to enable and disable PGE, but we don't care. */ +static unsigned long lguest_read_cr4(void) +{ + return 0; +} + +static void lguest_write_cr4(unsigned long val) +{ +} + +/* + * Page Table Handling. + * + * Now would be a good time to take a rest and grab a coffee or similarly + * relaxing stimulant. The easy parts are behind us, and the trek gradually + * winds uphill from here. + * + * Quick refresher: memory is divided into "pages" of 4096 bytes each. The CPU + * maps virtual addresses to physical addresses using "page tables". We could + * use one huge index of 1 million entries: each address is 4 bytes, so that's + * 1024 pages just to hold the page tables. But since most virtual addresses + * are unused, we use a two level index which saves space. The CR3 register + * contains the physical address of the top level "page directory" page, which + * contains physical addresses of up to 1024 second-level pages. Each of these + * second level pages contains up to 1024 physical addresses of actual pages, + * or Page Table Entries (PTEs). + * + * Here's a diagram, where arrows indicate physical addresses: + * + * CR3 ---> +---------+ + * | --------->+---------+ + * | | | PADDR1 | + * Top-level | | PADDR2 | + * (PMD) page | | | + * | | Lower-level | + * | | (PTE) page | + * | | | | + * .... .... + * + * So to convert a virtual address to a physical address, we look up the top + * level, which points us to the second level, which gives us the physical + * address of that page. If the top level entry was not present, or the second + * level entry was not present, then the virtual address is invalid (we + * say "the page was not mapped"). + * + * Put another way, a 32-bit virtual address is divided up like so: + * + * 1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 + * |<---- 10 bits ---->|<---- 10 bits ---->|<------ 12 bits ------>| + * Index into top Index into second Offset within page + * page directory page pagetable page + * + * The kernel spends a lot of time changing both the top-level page directory + * and lower-level pagetable pages. The Guest doesn't know physical addresses, + * so while it maintains these page tables exactly like normal, it also needs + * to keep the Host informed whenever it makes a change: the Host will create + * the real page tables based on the Guests'. + */ + +/* The Guest calls this to set a second-level entry (pte), ie. to map a page + * into a process' address space. We set the entry then tell the Host the + * toplevel and address this corresponds to. The Guest uses one pagetable per + * process, so we need to tell the Host which one we're changing (mm->pgd). */ +static void lguest_set_pte_at(struct mm_struct *mm, unsigned long addr, + pte_t *ptep, pte_t pteval) +{ + *ptep = pteval; + lazy_hcall(LHCALL_SET_PTE, __pa(mm->pgd), addr, pteval.pte_low); +} + +/* The Guest calls this to set a top-level entry. Again, we set the entry then + * tell the Host which top-level page we changed, and the index of the entry we + * changed. */ +static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval) +{ + *pmdp = pmdval; + lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK, + (__pa(pmdp)&(PAGE_SIZE-1))/4, 0); +} + +/* There are a couple of legacy places where the kernel sets a PTE, but we + * don't know the top level any more. This is useless for us, since we don't + * know which pagetable is changing or what address, so we just tell the Host + * to forget all of them. Fortunately, this is very rare. + * + * ... except in early boot when the kernel sets up the initial pagetables, + * which makes booting astonishingly slow. So we don't even tell the Host + * anything changed until we've done the first page table switch. + */ +static void lguest_set_pte(pte_t *ptep, pte_t pteval) +{ + *ptep = pteval; + /* Don't bother with hypercall before initial setup. */ + if (current_cr3) + lazy_hcall(LHCALL_FLUSH_TLB, 1, 0, 0); +} + +/* Unfortunately for Lguest, the pv_mmu_ops for page tables were based on + * native page table operations. On native hardware you can set a new page + * table entry whenever you want, but if you want to remove one you have to do + * a TLB flush (a TLB is a little cache of page table entries kept by the CPU). + * + * So the lguest_set_pte_at() and lguest_set_pmd() functions above are only + * called when a valid entry is written, not when it's removed (ie. marked not + * present). Instead, this is where we come when the Guest wants to remove a + * page table entry: we tell the Host to set that entry to 0 (ie. the present + * bit is zero). */ +static void lguest_flush_tlb_single(unsigned long addr) +{ + /* Simply set it to zero: if it was not, it will fault back in. */ + lazy_hcall(LHCALL_SET_PTE, current_cr3, addr, 0); +} + +/* This is what happens after the Guest has removed a large number of entries. + * This tells the Host that any of the page table entries for userspace might + * have changed, ie. virtual addresses below PAGE_OFFSET. */ +static void lguest_flush_tlb_user(void) +{ + lazy_hcall(LHCALL_FLUSH_TLB, 0, 0, 0); +} + +/* This is called when the kernel page tables have changed. That's not very + * common (unless the Guest is using highmem, which makes the Guest extremely + * slow), so it's worth separating this from the user flushing above. */ +static void lguest_flush_tlb_kernel(void) +{ + lazy_hcall(LHCALL_FLUSH_TLB, 1, 0, 0); +} + +/* + * The Unadvanced Programmable Interrupt Controller. + * + * This is an attempt to implement the simplest possible interrupt controller. + * I spent some time looking though routines like set_irq_chip_and_handler, + * set_irq_chip_and_handler_name, set_irq_chip_data and set_phasers_to_stun and + * I *think* this is as simple as it gets. + * + * We can tell the Host what interrupts we want blocked ready for using the + * lguest_data.interrupts bitmap, so disabling (aka "masking") them is as + * simple as setting a bit. We don't actually "ack" interrupts as such, we + * just mask and unmask them. I wonder if we should be cleverer? + */ +static void disable_lguest_irq(unsigned int irq) +{ + set_bit(irq, lguest_data.blocked_interrupts); +} + +static void enable_lguest_irq(unsigned int irq) +{ + clear_bit(irq, lguest_data.blocked_interrupts); +} + +/* This structure describes the lguest IRQ controller. */ +static struct irq_chip lguest_irq_controller = { + .name = "lguest", + .mask = disable_lguest_irq, + .mask_ack = disable_lguest_irq, + .unmask = enable_lguest_irq, +}; + +/* This sets up the Interrupt Descriptor Table (IDT) entry for each hardware + * interrupt (except 128, which is used for system calls), and then tells the + * Linux infrastructure that each interrupt is controlled by our level-based + * lguest interrupt controller. */ +static void __init lguest_init_IRQ(void) +{ + unsigned int i; + + for (i = 0; i < LGUEST_IRQS; i++) { + int vector = FIRST_EXTERNAL_VECTOR + i; + if (vector != SYSCALL_VECTOR) { + set_intr_gate(vector, interrupt[i]); + set_irq_chip_and_handler(i, &lguest_irq_controller, + handle_level_irq); + } + } + /* This call is required to set up for 4k stacks, where we have + * separate stacks for hard and soft interrupts. */ + irq_ctx_init(smp_processor_id()); +} + +/* + * Time. + * + * It would be far better for everyone if the Guest had its own clock, but + * until then the Host gives us the time on every interrupt. + */ +static unsigned long lguest_get_wallclock(void) +{ + return lguest_data.time.tv_sec; +} + +static cycle_t lguest_clock_read(void) +{ + unsigned long sec, nsec; + + /* If the Host tells the TSC speed, we can trust that. */ + if (lguest_data.tsc_khz) + return native_read_tsc(); + + /* If we can't use the TSC, we read the time value written by the Host. + * Since it's in two parts (seconds and nanoseconds), we risk reading + * it just as it's changing from 99 & 0.999999999 to 100 and 0, and + * getting 99 and 0. As Linux tends to come apart under the stress of + * time travel, we must be careful: */ + do { + /* First we read the seconds part. */ + sec = lguest_data.time.tv_sec; + /* This read memory barrier tells the compiler and the CPU that + * this can't be reordered: we have to complete the above + * before going on. */ + rmb(); + /* Now we read the nanoseconds part. */ + nsec = lguest_data.time.tv_nsec; + /* Make sure we've done that. */ + rmb(); + /* Now if the seconds part has changed, try again. */ + } while (unlikely(lguest_data.time.tv_sec != sec)); + + /* Our non-TSC clock is in real nanoseconds. */ + return sec*1000000000ULL + nsec; +} + +/* This is what we tell the kernel is our clocksource. */ +static struct clocksource lguest_clock = { + .name = "lguest", + .rating = 400, + .read = lguest_clock_read, + .mask = CLOCKSOURCE_MASK(64), + .mult = 1 << 22, + .shift = 22, + .flags = CLOCK_SOURCE_IS_CONTINUOUS, +}; + +/* The "scheduler clock" is just our real clock, adjusted to start at zero */ +static unsigned long long lguest_sched_clock(void) +{ + return cyc2ns(&lguest_clock, lguest_clock_read() - clock_base); +} + +/* We also need a "struct clock_event_device": Linux asks us to set it to go + * off some time in the future. Actually, James Morris figured all this out, I + * just applied the patch. */ +static int lguest_clockevent_set_next_event(unsigned long delta, + struct clock_event_device *evt) +{ + if (delta < LG_CLOCK_MIN_DELTA) { + if (printk_ratelimit()) + printk(KERN_DEBUG "%s: small delta %lu ns\n", + __FUNCTION__, delta); + return -ETIME; + } + hcall(LHCALL_SET_CLOCKEVENT, delta, 0, 0); + return 0; +} + +static void lguest_clockevent_set_mode(enum clock_event_mode mode, + struct clock_event_device *evt) +{ + switch (mode) { + case CLOCK_EVT_MODE_UNUSED: + case CLOCK_EVT_MODE_SHUTDOWN: + /* A 0 argument shuts the clock down. */ + hcall(LHCALL_SET_CLOCKEVENT, 0, 0, 0); + break; + case CLOCK_EVT_MODE_ONESHOT: + /* This is what we expect. */ + break; + case CLOCK_EVT_MODE_PERIODIC: + BUG(); + case CLOCK_EVT_MODE_RESUME: + break; + } +} + +/* This describes our primitive timer chip. */ +static struct clock_event_device lguest_clockevent = { + .name = "lguest", + .features = CLOCK_EVT_FEAT_ONESHOT, + .set_next_event = lguest_clockevent_set_next_event, + .set_mode = lguest_clockevent_set_mode, + .rating = INT_MAX, + .mult = 1, + .shift = 0, + .min_delta_ns = LG_CLOCK_MIN_DELTA, + .max_delta_ns = LG_CLOCK_MAX_DELTA, +}; + +/* This is the Guest timer interrupt handler (hardware interrupt 0). We just + * call the clockevent infrastructure and it does whatever needs doing. */ +static void lguest_time_irq(unsigned int irq, struct irq_desc *desc) +{ + unsigned long flags; + + /* Don't interrupt us while this is running. */ + local_irq_save(flags); + lguest_clockevent.event_handler(&lguest_clockevent); + local_irq_restore(flags); +} + +/* At some point in the boot process, we get asked to set up our timing + * infrastructure. The kernel doesn't expect timer interrupts before this, but + * we cleverly initialized the "blocked_interrupts" field of "struct + * lguest_data" so that timer interrupts were blocked until now. */ +static void lguest_time_init(void) +{ + /* Set up the timer interrupt (0) to go to our simple timer routine */ + set_irq_handler(0, lguest_time_irq); + + /* Our clock structure look like arch/i386/kernel/tsc.c if we can use + * the TSC, otherwise it's a dumb nanosecond-resolution clock. Either + * way, the "rating" is initialized so high that it's always chosen + * over any other clocksource. */ + if (lguest_data.tsc_khz) + lguest_clock.mult = clocksource_khz2mult(lguest_data.tsc_khz, + lguest_clock.shift); + clock_base = lguest_clock_read(); + clocksource_register(&lguest_clock); + + /* Now we've set up our clock, we can use it as the scheduler clock */ + pv_time_ops.sched_clock = lguest_sched_clock; + + /* We can't set cpumask in the initializer: damn C limitations! Set it + * here and register our timer device. */ + lguest_clockevent.cpumask = cpumask_of_cpu(0); + clockevents_register_device(&lguest_clockevent); + + /* Finally, we unblock the timer interrupt. */ + enable_lguest_irq(0); +} + +/* + * Miscellaneous bits and pieces. + * + * Here is an oddball collection of functions which the Guest needs for things + * to work. They're pretty simple. + */ + +/* The Guest needs to tell the host what stack it expects traps to use. For + * native hardware, this is part of the Task State Segment mentioned above in + * lguest_load_tr_desc(), but to help hypervisors there's this special call. + * + * We tell the Host the segment we want to use (__KERNEL_DS is the kernel data + * segment), the privilege level (we're privilege level 1, the Host is 0 and + * will not tolerate us trying to use that), the stack pointer, and the number + * of pages in the stack. */ +static void lguest_load_esp0(struct tss_struct *tss, + struct thread_struct *thread) +{ + lazy_hcall(LHCALL_SET_STACK, __KERNEL_DS|0x1, thread->esp0, + THREAD_SIZE/PAGE_SIZE); +} + +/* Let's just say, I wouldn't do debugging under a Guest. */ +static void lguest_set_debugreg(int regno, unsigned long value) +{ + /* FIXME: Implement */ +} + +/* There are times when the kernel wants to make sure that no memory writes are + * caught in the cache (that they've all reached real hardware devices). This + * doesn't matter for the Guest which has virtual hardware. + * + * On the Pentium 4 and above, cpuid() indicates that the Cache Line Flush + * (clflush) instruction is available and the kernel uses that. Otherwise, it + * uses the older "Write Back and Invalidate Cache" (wbinvd) instruction. + * Unlike clflush, wbinvd can only be run at privilege level 0. So we can + * ignore clflush, but replace wbinvd. + */ +static void lguest_wbinvd(void) +{ +} + +/* If the Guest expects to have an Advanced Programmable Interrupt Controller, + * we play dumb by ignoring writes and returning 0 for reads. So it's no + * longer Programmable nor Controlling anything, and I don't think 8 lines of + * code qualifies for Advanced. It will also never interrupt anything. It + * does, however, allow us to get through the Linux boot code. */ +#ifdef CONFIG_X86_LOCAL_APIC +static void lguest_apic_write(unsigned long reg, unsigned long v) +{ +} + +static unsigned long lguest_apic_read(unsigned long reg) +{ + return 0; +} +#endif + +/* STOP! Until an interrupt comes in. */ +static void lguest_safe_halt(void) +{ + hcall(LHCALL_HALT, 0, 0, 0); +} + +/* Perhaps CRASH isn't the best name for this hypercall, but we use it to get a + * message out when we're crashing as well as elegant termination like powering + * off. + * + * Note that the Host always prefers that the Guest speak in physical addresses + * rather than virtual addresses, so we use __pa() here. */ +static void lguest_power_off(void) +{ + hcall(LHCALL_CRASH, __pa("Power down"), 0, 0); +} + +/* + * Panicing. + * + * Don't. But if you did, this is what happens. + */ +static int lguest_panic(struct notifier_block *nb, unsigned long l, void *p) +{ + hcall(LHCALL_CRASH, __pa(p), 0, 0); + /* The hcall won't return, but to keep gcc happy, we're "done". */ + return NOTIFY_DONE; +} + +static struct notifier_block paniced = { + .notifier_call = lguest_panic +}; + +/* Setting up memory is fairly easy. */ +static __init char *lguest_memory_setup(void) +{ + /* We do this here and not earlier because lockcheck barfs if we do it + * before start_kernel() */ + atomic_notifier_chain_register(&panic_notifier_list, &paniced); + + /* The Linux bootloader header contains an "e820" memory map: the + * Launcher populated the first entry with our memory limit. */ + add_memory_region(boot_params.e820_map[0].addr, + boot_params.e820_map[0].size, + boot_params.e820_map[0].type); + + /* This string is for the boot messages. */ + return "LGUEST"; +} + +/*G:050 + * Patching (Powerfully Placating Performance Pedants) + * + * We have already seen that pv_ops structures let us replace simple + * native instructions with calls to the appropriate back end all throughout + * the kernel. This allows the same kernel to run as a Guest and as a native + * kernel, but it's slow because of all the indirect branches. + * + * Remember that David Wheeler quote about "Any problem in computer science can + * be solved with another layer of indirection"? The rest of that quote is + * "... But that usually will create another problem." This is the first of + * those problems. + * + * Our current solution is to allow the paravirt back end to optionally patch + * over the indirect calls to replace them with something more efficient. We + * patch the four most commonly called functions: disable interrupts, enable + * interrupts, restore interrupts and save interrupts. We usually have 10 + * bytes to patch into: the Guest versions of these operations are small enough + * that we can fit comfortably. + * + * First we need assembly templates of each of the patchable Guest operations, + * and these are in lguest_asm.S. */ + +/*G:060 We construct a table from the assembler templates: */ +static const struct lguest_insns +{ + const char *start, *end; +} lguest_insns[] = { + [PARAVIRT_PATCH(pv_irq_ops.irq_disable)] = { lgstart_cli, lgend_cli }, + [PARAVIRT_PATCH(pv_irq_ops.irq_enable)] = { lgstart_sti, lgend_sti }, + [PARAVIRT_PATCH(pv_irq_ops.restore_fl)] = { lgstart_popf, lgend_popf }, + [PARAVIRT_PATCH(pv_irq_ops.save_fl)] = { lgstart_pushf, lgend_pushf }, +}; + +/* Now our patch routine is fairly simple (based on the native one in + * paravirt.c). If we have a replacement, we copy it in and return how much of + * the available space we used. */ +static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf, + unsigned long addr, unsigned len) +{ + unsigned int insn_len; + + /* Don't do anything special if we don't have a replacement */ + if (type >= ARRAY_SIZE(lguest_insns) || !lguest_insns[type].start) + return paravirt_patch_default(type, clobber, ibuf, addr, len); + + insn_len = lguest_insns[type].end - lguest_insns[type].start; + + /* Similarly if we can't fit replacement (shouldn't happen, but let's + * be thorough). */ + if (len < insn_len) + return paravirt_patch_default(type, clobber, ibuf, addr, len); + + /* Copy in our instructions. */ + memcpy(ibuf, lguest_insns[type].start, insn_len); + return insn_len; +} + +/*G:030 Once we get to lguest_init(), we know we're a Guest. The pv_ops + * structures in the kernel provide points for (almost) every routine we have + * to override to avoid privileged instructions. */ +__init void lguest_init(void *boot) +{ + /* Copy boot parameters first: the Launcher put the physical location + * in %esi, and head.S converted that to a virtual address and handed + * it to us. We use "__memcpy" because "memcpy" sometimes tries to do + * tricky things to go faster, and we're not ready for that. */ + __memcpy(&boot_params, boot, PARAM_SIZE); + /* The boot parameters also tell us where the command-line is: save + * that, too. */ + __memcpy(boot_command_line, __va(boot_params.hdr.cmd_line_ptr), + COMMAND_LINE_SIZE); + + /* We're under lguest, paravirt is enabled, and we're running at + * privilege level 1, not 0 as normal. */ + pv_info.name = "lguest"; + pv_info.paravirt_enabled = 1; + pv_info.kernel_rpl = 1; + + /* We set up all the lguest overrides for sensitive operations. These + * are detailed with the operations themselves. */ + + /* interrupt-related operations */ + pv_irq_ops.init_IRQ = lguest_init_IRQ; + pv_irq_ops.save_fl = save_fl; + pv_irq_ops.restore_fl = restore_fl; + pv_irq_ops.irq_disable = irq_disable; + pv_irq_ops.irq_enable = irq_enable; + pv_irq_ops.safe_halt = lguest_safe_halt; + + /* init-time operations */ + pv_init_ops.memory_setup = lguest_memory_setup; + pv_init_ops.patch = lguest_patch; + + /* Intercepts of various cpu instructions */ + pv_cpu_ops.load_gdt = lguest_load_gdt; + pv_cpu_ops.cpuid = lguest_cpuid; + pv_cpu_ops.load_idt = lguest_load_idt; + pv_cpu_ops.iret = lguest_iret; + pv_cpu_ops.load_esp0 = lguest_load_esp0; + pv_cpu_ops.load_tr_desc = lguest_load_tr_desc; + pv_cpu_ops.set_ldt = lguest_set_ldt; + pv_cpu_ops.load_tls = lguest_load_tls; + pv_cpu_ops.set_debugreg = lguest_set_debugreg; + pv_cpu_ops.clts = lguest_clts; + pv_cpu_ops.read_cr0 = lguest_read_cr0; + pv_cpu_ops.write_cr0 = lguest_write_cr0; + pv_cpu_ops.read_cr4 = lguest_read_cr4; + pv_cpu_ops.write_cr4 = lguest_write_cr4; + pv_cpu_ops.write_gdt_entry = lguest_write_gdt_entry; + pv_cpu_ops.write_idt_entry = lguest_write_idt_entry; + pv_cpu_ops.wbinvd = lguest_wbinvd; + pv_cpu_ops.lazy_mode.enter = paravirt_enter_lazy_cpu; + pv_cpu_ops.lazy_mode.leave = lguest_leave_lazy_mode; + + /* pagetable management */ + pv_mmu_ops.write_cr3 = lguest_write_cr3; + pv_mmu_ops.flush_tlb_user = lguest_flush_tlb_user; + pv_mmu_ops.flush_tlb_single = lguest_flush_tlb_single; + pv_mmu_ops.flush_tlb_kernel = lguest_flush_tlb_kernel; + pv_mmu_ops.set_pte = lguest_set_pte; + pv_mmu_ops.set_pte_at = lguest_set_pte_at; + pv_mmu_ops.set_pmd = lguest_set_pmd; + pv_mmu_ops.read_cr2 = lguest_read_cr2; + pv_mmu_ops.read_cr3 = lguest_read_cr3; + pv_mmu_ops.lazy_mode.enter = paravirt_enter_lazy_mmu; + pv_mmu_ops.lazy_mode.leave = lguest_leave_lazy_mode; + +#ifdef CONFIG_X86_LOCAL_APIC + /* apic read/write intercepts */ + pv_apic_ops.apic_write = lguest_apic_write; + pv_apic_ops.apic_write_atomic = lguest_apic_write; + pv_apic_ops.apic_read = lguest_apic_read; +#endif + + /* time operations */ + pv_time_ops.get_wallclock = lguest_get_wallclock; + pv_time_ops.time_init = lguest_time_init; + + /* Now is a good time to look at the implementations of these functions + * before returning to the rest of lguest_init(). */ + + /*G:070 Now we've seen all the paravirt_ops, we return to + * lguest_init() where the rest of the fairly chaotic boot setup + * occurs. + * + * The Host expects our first hypercall to tell it where our "struct + * lguest_data" is, so we do that first. */ + hcall(LHCALL_LGUEST_INIT, __pa(&lguest_data), 0, 0); + + /* The native boot code sets up initial page tables immediately after + * the kernel itself, and sets init_pg_tables_end so they're not + * clobbered. The Launcher places our initial pagetables somewhere at + * the top of our physical memory, so we don't need extra space: set + * init_pg_tables_end to the end of the kernel. */ + init_pg_tables_end = __pa(pg0); + + /* Load the %fs segment register (the per-cpu segment register) with + * the normal data segment to get through booting. */ + asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_DS) : "memory"); + + /* Clear the part of the kernel data which is expected to be zero. + * Normally it will be anyway, but if we're loading from a bzImage with + * CONFIG_RELOCATALE=y, the relocations will be sitting here. */ + memset(__bss_start, 0, __bss_stop - __bss_start); + + /* The Host uses the top of the Guest's virtual address space for the + * Host<->Guest Switcher, and it tells us how much it needs in + * lguest_data.reserve_mem, set up on the LGUEST_INIT hypercall. */ + reserve_top_address(lguest_data.reserve_mem); + + /* If we don't initialize the lock dependency checker now, it crashes + * paravirt_disable_iospace. */ + lockdep_init(); + + /* The IDE code spends about 3 seconds probing for disks: if we reserve + * all the I/O ports up front it can't get them and so doesn't probe. + * Other device drivers are similar (but less severe). This cuts the + * kernel boot time on my machine from 4.1 seconds to 0.45 seconds. */ + paravirt_disable_iospace(); + + /* This is messy CPU setup stuff which the native boot code does before + * start_kernel, so we have to do, too: */ + cpu_detect(&new_cpu_data); + /* head.S usually sets up the first capability word, so do it here. */ + new_cpu_data.x86_capability[0] = cpuid_edx(1); + + /* Math is always hard! */ + new_cpu_data.hard_math = 1; + +#ifdef CONFIG_X86_MCE + mce_disabled = 1; +#endif +#ifdef CONFIG_ACPI + acpi_disabled = 1; + acpi_ht = 0; +#endif + + /* We set the perferred console to "hvc". This is the "hypervisor + * virtual console" driver written by the PowerPC people, which we also + * adapted for lguest's use. */ + add_preferred_console("hvc", 0, NULL); + + /* Last of all, we set the power management poweroff hook to point to + * the Guest routine to power off. */ + pm_power_off = lguest_power_off; + + /* Now we're set up, call start_kernel() in init/main.c and we proceed + * to boot as normal. It never returns. */ + start_kernel(); +} +/* + * This marks the end of stage II of our journey, The Guest. + * + * It is now time for us to explore the nooks and crannies of the three Guest + * devices and complete our understanding of the Guest in "make Drivers". + */ diff --git a/arch/x86/lguest/i386_head.S b/arch/x86/lguest/i386_head.S new file mode 100644 index 00000000000..6d7a74f07c4 --- /dev/null +++ b/arch/x86/lguest/i386_head.S @@ -0,0 +1,93 @@ +#include <linux/linkage.h> +#include <linux/lguest.h> +#include <asm/asm-offsets.h> +#include <asm/thread_info.h> +#include <asm/processor-flags.h> + +/*G:020 This is where we begin: we have a magic signature which the launcher + * looks for. The plan is that the Linux boot protocol will be extended with a + * "platform type" field which will guide us here from the normal entry point, + * but for the moment this suffices. The normal boot code uses %esi for the + * boot header, so we do too. We convert it to a virtual address by adding + * PAGE_OFFSET, and hand it to lguest_init() as its argument (ie. %eax). + * + * The .section line puts this code in .init.text so it will be discarded after + * boot. */ +.section .init.text, "ax", @progbits +.ascii "GenuineLguest" + /* Set up initial stack. */ + movl $(init_thread_union+THREAD_SIZE),%esp + movl %esi, %eax + addl $__PAGE_OFFSET, %eax + jmp lguest_init + +/*G:055 We create a macro which puts the assembler code between lgstart_ and + * lgend_ markers. These templates are put in the .text section: they can't be + * discarded after boot as we may need to patch modules, too. */ +.text +#define LGUEST_PATCH(name, insns...) \ + lgstart_##name: insns; lgend_##name:; \ + .globl lgstart_##name; .globl lgend_##name + +LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled) +LGUEST_PATCH(sti, movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled) +LGUEST_PATCH(popf, movl %eax, lguest_data+LGUEST_DATA_irq_enabled) +LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax) +/*:*/ + +/* These demark the EIP range where host should never deliver interrupts. */ +.global lguest_noirq_start +.global lguest_noirq_end + +/*M:004 When the Host reflects a trap or injects an interrupt into the Guest, + * it sets the eflags interrupt bit on the stack based on + * lguest_data.irq_enabled, so the Guest iret logic does the right thing when + * restoring it. However, when the Host sets the Guest up for direct traps, + * such as system calls, the processor is the one to push eflags onto the + * stack, and the interrupt bit will be 1 (in reality, interrupts are always + * enabled in the Guest). + * + * This turns out to be harmless: the only trap which should happen under Linux + * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc + * regions), which has to be reflected through the Host anyway. If another + * trap *does* go off when interrupts are disabled, the Guest will panic, and + * we'll never get to this iret! :*/ + +/*G:045 There is one final paravirt_op that the Guest implements, and glancing + * at it you can see why I left it to last. It's *cool*! It's in *assembler*! + * + * The "iret" instruction is used to return from an interrupt or trap. The + * stack looks like this: + * old address + * old code segment & privilege level + * old processor flags ("eflags") + * + * The "iret" instruction pops those values off the stack and restores them all + * at once. The only problem is that eflags includes the Interrupt Flag which + * the Guest can't change: the CPU will simply ignore it when we do an "iret". + * So we have to copy eflags from the stack to lguest_data.irq_enabled before + * we do the "iret". + * + * There are two problems with this: firstly, we need to use a register to do + * the copy and secondly, the whole thing needs to be atomic. The first + * problem is easy to solve: push %eax on the stack so we can use it, and then + * restore it at the end just before the real "iret". + * + * The second is harder: copying eflags to lguest_data.irq_enabled will turn + * interrupts on before we're finished, so we could be interrupted before we + * return to userspace or wherever. Our solution to this is to surround the + * code with lguest_noirq_start: and lguest_noirq_end: labels. We tell the + * Host that it is *never* to interrupt us there, even if interrupts seem to be + * enabled. */ +ENTRY(lguest_iret) + pushl %eax + movl 12(%esp), %eax +lguest_noirq_start: + /* Note the %ss: segment prefix here. Normal data accesses use the + * "ds" segment, but that will have already been restored for whatever + * we're returning to (such as userspace): we can't trust it. The %ss: + * prefix makes sure we use the stack segment, which is still valid. */ + movl %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled + popl %eax + iret +lguest_noirq_end: |