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);
+       /*
+        * 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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