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authorJeremy Fitzhardinge <jeremy@xensource.com>2007-07-17 18:37:06 -0700
committerJeremy Fitzhardinge <jeremy@goop.org>2007-07-18 08:47:44 -0700
commit8b84ad942b534f8faeb34b68f0f7277ea375fed0 (patch)
tree911472add5b9068a95a77215a70dac63edf6202e /arch/i386/xen/enlighten.c
parentd66bf8fcf3fce058a1cd164a7c8ee6093fdf039c (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/enlighten.c')
-rw-r--r--arch/i386/xen/enlighten.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/arch/i386/xen/enlighten.c b/arch/i386/xen/enlighten.c
index 031dc1dcf819..42756771b8eb 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,

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