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

[patchbot@xxxxxxx]

commit f6790389613cd54775ece6575013a679572b46b3
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Tue Sep 22 20:31:04 2020 +0100
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Mon Sep 28 18:42:54 2020 +0100

    xen/arm: sched: Ensure the vCPU context is seen before vcpu_pause() returns
    
    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>
    Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
    Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 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 9258f6d3fa..3b37f899b9 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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