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 <[EMAIL PROTECTED]>
===================================================================
--- 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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