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Subject[PATCH 48 of 55] xen64: Clear %fs on xen_load_tls()
Date
From
From: Eduardo Habkost <ehabkost@redhat.com>

We need to do this, otherwise we can get a GPF on hypercall return
after TLS descriptor is cleared but %fs is still pointing to it.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
arch/x86/xen/enlighten.c | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -364,14 +364,6 @@

static void xen_load_tls(struct thread_struct *t, unsigned int cpu)
{
- xen_mc_batch();
-
- load_TLS_descriptor(t, cpu, 0);
- load_TLS_descriptor(t, cpu, 1);
- 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
@@ -380,9 +372,30 @@
* 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.
+ *
+ * On x86_64, this hack is not used for %gs, because gs points
+ * to KERNEL_GS_BASE (and uses it for PDA references), so we
+ * must not zero %gs on x86_64
+ *
+ * For x86_64, we need to zero %fs, otherwise we may get an
+ * exception between the new %fs descriptor being loaded and
+ * %fs being effectively cleared at __switch_to().
*/
- if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU)
+ if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU) {
+#ifdef CONFIG_X86_32
loadsegment(gs, 0);
+#else
+ loadsegment(fs, 0);
+#endif
+ }
+
+ xen_mc_batch();
+
+ load_TLS_descriptor(t, cpu, 0);
+ load_TLS_descriptor(t, cpu, 1);
+ load_TLS_descriptor(t, cpu, 2);
+
+ xen_mc_issue(PARAVIRT_LAZY_CPU);
}

#ifdef CONFIG_X86_64



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