[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [RESEND][PATCH] xen/arm: sched: Ensure the vCPU context is seen before vcpu_pause() returns
On Wed, 23 Sep 2020, Bertrand Marquis wrote: > Hi, > > > On 22 Sep 2020, at 20:31, Julien Grall <julien@xxxxxxx> wrote: > > > > 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> > Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx> Acked-by: Stefano Stabellini <sstabellini@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) > > \ > > -- > > 2.17.1 > > >
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