[PATCH] xen/arm: sched: Ensure the vCPU context is seen before vcpu_pause() returns

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

Some callers of vcpu_pause() will expect to access the latest vcpu
context when the function returns (see XENDOMCTL_{set,get}vcpucontext}.

However, the latest vCPU context can only be observed after
v->is_running has been observed to be false.

As there is no memory barrier instruction generated, a processor could
try to speculatively access the vCPU context before it was observed.

To prevent the corruption of the vCPU context, we need to insert a
memory barrier instruction after v->is_running is observed and before
the context is accessed. This barrier is added in sync_vcpu_execstate()
as it seems to be the place where we expect the synchronization to
happen.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

---

Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Dario Faggioli <dfaggioli@xxxxxxxx>
Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>

I am also adding the x86 and scheduler maintainers because I am not sure
whether this barrier should be part of the common code instead.
---
 xen/arch/arm/domain.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 9258f6d3faa2..3b37f899b9da 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -371,7 +371,20 @@ void sync_local_execstate(void)
 
 void sync_vcpu_execstate(struct vcpu *v)
 {
-    /* Nothing to do -- no lazy switching */
+    /*
+     * We don't support lazy switching.
+     *
+     * However the context may have been saved from a remote pCPU so we
+     * need a barrier to ensure it is observed before continuing.
+     *
+     * Per vcpu_context_saved(), the context can be observed when
+     * v->is_running is false (the caller should check it before calling
+     * this function).
+     *
+     * Note this is a full barrier to also prevent update of the context
+     * to happen before it was observed.
+     */
+    smp_mb();
 }
 
 #define NEXT_ARG(fmt, args)                                                 \
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2.17.1