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[Xen-devel] [patch 17/24] Xen-paravirt_ops: avoid having a bad selector in %gs during context switch

During a context switch from a usermode process (which has a TLS
segment set up, and %gs pointing to it) to a kernel process (which has
no TLS and a null %gs selector), there's a period when %gs contains a
stale selector value after updating the GDT.  This causes Xen to
hiccup because on returning from the GDT update hypercall as it tries
to reload %gs and suffers another fault.

This patch words around this by zeroing %gs during the context switch
so that it doesn't cause problems, but the real fix is to change Xen
to avoid touching %gs when it doesn't need to.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>

--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -719,6 +719,14 @@ struct task_struct fastcall * __switch_t
        savesegment(gs, prev->gs);
+        * Temporary hack: zero gs now that we've saved it so that Xen
+        * doesn't try to reload the old value after changing the GDT
+        * during the context switch.  This can go away once Xen has
+        * been taught to only reload %gs when it absolutely must.
+        */
+       loadsegment(gs, 0);
+       /*
         * Load the per-thread Thread-Local Storage descriptor.
        load_TLS(next, cpu);


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