Commit 8b84ad94 authored by Jeremy Fitzhardinge's avatar Jeremy Fitzhardinge Committed by Jeremy Fitzhardinge
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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: default avatarJeremy Fitzhardinge <>
Signed-off-by: default avatarChris Wright <>
parent d66bf8fc
......@@ -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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