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xen: hack to prevent bad segment register reload

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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>
This commit is contained in:
Jeremy Fitzhardinge 2007-07-17 18:37:06 -07:00 committed by Jeremy Fitzhardinge
parent d66bf8fcf3
commit 8b84ad942b
1 changed files with 12 additions and 0 deletions
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arch/i386/xen/enlighten.c
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@ -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,