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

[Julien Grall <julien@xxxxxxx>]

On 23/09/2020 12:08, Bertrand Marquis wrote:
> Hi Julien,
>
> > 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.
> The function vcpu_context_saved does contain a memory barrier already.
Memory barriers usually work in pair. We have a write barrier in 
vcpu_context_saved() but no read barrier in the code relying on the 
v->is_running.
> Shouldn’t we make sure instead that any time is_running is modified to
> false there is a barrier before (which is the case in vcpu_context_saved) ?
>
> I understand the goal here but the barrier seem very far from the modification
> of is_running.
That's not what I am trying to fix (see above). Instead, this patch will 
ensure that when a pCPU observe v->is_running = false, then it can rely 
on the context of the vCPU to be valid.
Cheers,

--
Julien Grall