i386: hack to prevent bad segment register reload From: Jeremy Fitzhardinge 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 Signed-off-by: Andi Kleen --- arch/i386/xen/enlighten.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) =================================================================== Index: linux/arch/i386/xen/enlighten.c =================================================================== --- linux.orig/arch/i386/xen/enlighten.c +++ linux/arch/i386/xen/enlighten.c @@ -291,6 +291,18 @@ static void xen_load_tls(struct thread_s 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, u32 low, u32 high)