Doh, I forgot to CC Stefano on the e-mail. I will resend it.
Sorry for the inconvenience. On 22/09/2020 20:28, Julien Grall wrote:
From: Julien Grall <jgr...@amazon.com> 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 <jgr...@amazon.com> --- Cc: Andrew Cooper <andrew.coop...@citrix.com> Cc: Jan Beulich <jbeul...@suse.com> Cc: George Dunlap <george.dun...@citrix.com> Cc: Dario Faggioli <dfaggi...@suse.com> Cc: Bertrand Marquis <bertrand.marq...@arm.com> 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) \
-- Julien Grall
