IRQ: Maintain regs pointer globally rather than passing to IRQ handlers

Maintain a per-CPU global "struct pt_regs *" variable which can be used instead
of passing regs around manually through all ~1800 interrupt handlers in the
Linux kernel.

The regs pointer is used in few places, but it potentially costs both stack
space and code to pass it around.  On the FRV arch, removing the regs parameter
from all the genirq function results in a 20% speed up of the IRQ exit path
(ie: from leaving timer_interrupt() to leaving do_IRQ()).

Where appropriate, an arch may override the generic storage facility and do
something different with the variable.  On FRV, for instance, the address is
maintained in GR28 at all times inside the kernel as part of general exception
handling.

Having looked over the code, it appears that the parameter may be handed down
through up to twenty or so layers of functions.  Consider a USB character
device attached to a USB hub, attached to a USB controller that posts its
interrupts through a cascaded auxiliary interrupt controller.  A character
device driver may want to pass regs to the sysrq handler through the input
layer which adds another few layers of parameter passing.

I've build this code with allyesconfig for x86_64 and i386.  I've runtested the
main part of the code on FRV and i386, though I can't test most of the drivers.
I've also done partial conversion for powerpc and MIPS - these at least compile
with minimal configurations.

This will affect all archs.  Mostly the changes should be relatively easy.
Take do_IRQ(), store the regs pointer at the beginning, saving the old one:

	struct pt_regs *old_regs = set_irq_regs(regs);

And put the old one back at the end:

	set_irq_regs(old_regs);

Don't pass regs through to generic_handle_irq() or __do_IRQ().

In timer_interrupt(), this sort of change will be necessary:

	-	update_process_times(user_mode(regs));
	-	profile_tick(CPU_PROFILING, regs);
	+	update_process_times(user_mode(get_irq_regs()));
	+	profile_tick(CPU_PROFILING);

I'd like to move update_process_times()'s use of get_irq_regs() into itself,
except that i386, alone of the archs, uses something other than user_mode().

Some notes on the interrupt handling in the drivers:

 (*) input_dev() is now gone entirely.  The regs pointer is no longer stored in
     the input_dev struct.

 (*) finish_unlinks() in drivers/usb/host/ohci-q.c needs checking.  It does
     something different depending on whether it's been supplied with a regs
     pointer or not.

 (*) Various IRQ handler function pointers have been moved to type
     irq_handler_t.

Signed-Off-By: David Howells <dhowells@redhat.com>
(cherry picked from 1b16e7ac850969f38b375e511e3fa2f474a33867 commit)

2 files changed, 17 insertions, 11 deletions

diff --git a/arch/mips/kernel/irq.c b/arch/mips/kernel/irq.c
index d955aaefbb8..a00b0e7ab9b 100644
--- a/arch/mips/kernel/irq.c
+++ b/arch/mips/kernel/irq.c
@@ -55,13 +55,15 @@ unsigned long irq_hwmask[NR_IRQS];
  */
 asmlinkage unsigned int do_IRQ(unsigned int irq, struct pt_regs *regs)
 {
+	struct pt_regs *old_regs = set_irq_regs(regs);
 	irq_enter();
 
 	__DO_IRQ_SMTC_HOOK();
-	__do_IRQ(irq, regs);
+	__do_IRQ(irq);
 
 	irq_exit();
 
+	set_irq_regs(old_regs);
 	return 1;
 }
 
diff --git a/arch/mips/kernel/time.c b/arch/mips/kernel/time.c
index a8340802f2d..d349eb9e4ff 100644
--- a/arch/mips/kernel/time.c
+++ b/arch/mips/kernel/time.c
@@ -322,18 +322,18 @@ static long last_rtc_update;
  * a broadcasted inter-processor interrupt which itself is triggered
  * by the global timer interrupt.
  */
-void local_timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
+void local_timer_interrupt(int irq, void *dev_id)
 {
 	if (current->pid)
-		profile_tick(CPU_PROFILING, regs);
-	update_process_times(user_mode(regs));
+		profile_tick(CPU_PROFILING);
+	update_process_times(user_mode(get_irq_regs()));
 }
 
 /*
  * High-level timer interrupt service routines.  This function
  * is set as irqaction->handler and is invoked through do_IRQ.
  */
-irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
+irqreturn_t timer_interrupt(int irq, void *dev_id)
 {
 	unsigned long j;
 	unsigned int count;
@@ -419,23 +419,24 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 	 * In SMP mode, local_timer_interrupt() is invoked by appropriate
 	 * low-level local timer interrupt handler.
 	 */
-	local_timer_interrupt(irq, dev_id, regs);
+	local_timer_interrupt(irq, dev_id);
 
 	return IRQ_HANDLED;
 }
 
-int null_perf_irq(struct pt_regs *regs)
+int null_perf_irq(void)
 {
 	return 0;
 }
 
-int (*perf_irq)(struct pt_regs *regs) = null_perf_irq;
+int (*perf_irq)(void) = null_perf_irq;
 
 EXPORT_SYMBOL(null_perf_irq);
 EXPORT_SYMBOL(perf_irq);
 
 asmlinkage void ll_timer_interrupt(int irq, struct pt_regs *regs)
 {
+	struct pt_regs *old_regs = set_irq_regs(regs);
 	int r2 = cpu_has_mips_r2;
 
 	irq_enter();
@@ -448,27 +449,30 @@ asmlinkage void ll_timer_interrupt(int irq, struct pt_regs *regs)
 	 * performance counter interrupt handler anyway.
 	 */
 	if (!r2 || (read_c0_cause() & (1 << 26)))
-		if (perf_irq(regs))
+		if (perf_irq())
 			goto out;
 
 	/* we keep interrupt disabled all the time */
 	if (!r2 || (read_c0_cause() & (1 << 30)))
-		timer_interrupt(irq, NULL, regs);
+		timer_interrupt(irq, NULL);
 
 out:
 	irq_exit();
+	set_irq_regs(old_regs);
 }
 
 asmlinkage void ll_local_timer_interrupt(int irq, struct pt_regs *regs)
 {
+	struct pt_regs *old_regs = set_irq_regs(regs);
 	irq_enter();
 	if (smp_processor_id() != 0)
 		kstat_this_cpu.irqs[irq]++;
 
 	/* we keep interrupt disabled all the time */
-	local_timer_interrupt(irq, NULL, regs);
+	local_timer_interrupt(irq, NULL);
 
 	irq_exit();
+	set_irq_regs(old_regs);
 }
 
 /*