Re: [PATCH v2 06/12] perf/x86: implement cross-HT corruption bug workaround

[Jiri Olsa]

On Thu, Oct 23, 2014 at 10:01:08AM +0200, Stephane Eranian wrote:
> On Thu, Oct 23, 2014 at 9:19 AM, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> > On Wed, Oct 22, 2014 at 02:31:51PM +0200, Jiri Olsa wrote:
> >> On Thu, Oct 09, 2014 at 06:34:40PM +0200, Stephane Eranian wrote:
> >> > From: Maria Dimakopoulou <maria.n.dimakopoulou@xxxxxxxxx>

SNIP

> >> > +   for_each_set_bit(i, cx->idxmsk, X86_PMC_IDX_MAX) {
> >> > +           /*
> >> > +            * exclusive event in sibling counter
> >> > +            * our corresponding counter cannot be used
> >> > +            * regardless of our event
> >> > +            */
> >> > +           if (xl->state[i] == INTEL_EXCL_EXCLUSIVE)
> >> > +                   __clear_bit(i, cx->idxmsk);
> >>
> >> if we want to check sibling counter, shouldn't we check xlo->state[i] instead? like
> >>
> >>               if (xlo->state[i] == INTEL_EXCL_EXCLUSIVE)
> >>                       __clear_bit(i, cx->idxmsk);
> >>
> >>
> >> and also in condition below?
> >
> > any comment on this? I'm curious, because it'd enlighten me
> > on how this is supposed to work ;-)
> >
> > I dont understand why you update the sibling's counter state instead
> > of the current cpuc->excl_thread_id HT, like in intel_commit_scheduling
> > while you hold lock for the current HT state
> >
> > could you please comment, I must be missing something
> >
> Yes, it is a bit confusing. It comes down that what the state represents.
> Let me explain.
> 
> In get_constraints(), you compute the bitmask of possible counters by looking
> at your own state in states[tid] (xl).
> 
> In commit_constraint(), the scheduler has picked counters and you need to commit
> the changes. But you don't update your state, you update your
> sibling's state. Why?
> Because of the bug, what you use influences what the sibling can measure. So you
> update the sibling's state to reflect its new constraint. When the
> sibling calls get_constraint()
> it will harvest the new constraints automatically.
> 
> Example:
> 
> HT0 wants to program a MEM (corrupting) event, it gathers its
> constraints from get_constraints().
> The mask is, let's say, 0x3. The scheduler picks counter0. Then, in
> commit_constraints(), you need
> to mark *HT1*'s counter0 in exclusive mode, i.e., it cannot be used
> anymore on that thread.

ah, I translated the INTEL_EXCL_EXCLUSIVE state as "here's the exclusive
event, sibling HT is forbiden to schedule in this counter (slot)"

cool, thanks
jirka
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