tree 911472add5b9068a95a77215a70dac63edf6202e
parent d66bf8fcf3fce058a1cd164a7c8ee6093fdf039c
author Jeremy Fitzhardinge <jeremy@xensource.com> 1184722626 -0700
committer Jeremy Fitzhardinge <jeremy@goop.org> 1184773664 -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 <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>