diff options
| author | Jeremy Fitzhardinge <jeremy@xensource.com> | 2007-07-17 18:37:06 -0700 |
|---|---|---|
| committer | Jeremy Fitzhardinge <jeremy@goop.org> | 2007-07-18 08:47:44 -0700 |
| commit | 8b84ad942b534f8faeb34b68f0f7277ea375fed0 (patch) | |
| tree | 911472add5b9068a95a77215a70dac63edf6202e /arch/i386/xen | |
| parent | d66bf8fcf3fce058a1cd164a7c8ee6093fdf039c (diff) | |
xen: hack to prevent bad segment register reload
The hypervisor saves and restores the segment registers as part of the
state is saves while context switching. If, during a context switch,
the next process doesn't use the TLS segments, it invalidates the GDT
entry, causing the segment register reload to fault. This fault
effectively doubles the cost of a context switch.
This patch is a band-aid workaround which clears the usermode %gs
after it has been saved for the previous process, but before it gets
reloaded for the next, and it avoids having the hypervisor attempt to
erroneously reload it.
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Diffstat (limited to 'arch/i386/xen')
| -rw-r--r-- | arch/i386/xen/enlighten.c | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/arch/i386/xen/enlighten.c b/arch/i386/xen/enlighten.c index 031dc1dcf81..42756771b8e 100644 --- a/arch/i386/xen/enlighten.c +++ b/arch/i386/xen/enlighten.c @@ -291,6 +291,18 @@ static void xen_load_tls(struct thread_struct *t, unsigned int cpu) load_TLS_descriptor(t, cpu, 2); xen_mc_issue(PARAVIRT_LAZY_CPU); + + /* + * XXX sleazy hack: If we're being called in a lazy-cpu zone, + * it means we're in a context switch, and %gs has just been + * saved. This means we can zero it out to prevent faults on + * exit from the hypervisor if the next process has no %gs. + * Either way, it has been saved, and the new value will get + * loaded properly. This will go away as soon as Xen has been + * modified to not save/restore %gs for normal hypercalls. + */ + if (xen_get_lazy_mode() == PARAVIRT_LAZY_CPU) + loadsegment(gs, 0); } static void xen_write_ldt_entry(struct desc_struct *dt, int entrynum, |