Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=8b84ad942b534f8faeb34b68f0f7277ea375fed0
Commit: 8b84ad942b534f8faeb34b68f0f7277ea375fed0
Parent: d66bf8fcf3fce058a1cd164a7c8ee6093fdf039c
Author: Jeremy Fitzhardinge <[EMAIL PROTECTED]>
AuthorDate: Tue Jul 17 18:37:06 2007 -0700
Committer: Jeremy Fitzhardinge <[EMAIL PROTECTED]>
CommitDate: Wed Jul 18 08:47:44 2007 -0700
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 <[EMAIL PROTECTED]>
Signed-off-by: Chris Wright <[EMAIL PROTECTED]>
---
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 031dc1d..4275677 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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