Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

[Xu, Like]

On 2021/1/15 22:44, Peter Zijlstra wrote:
> On Fri, Jan 15, 2021 at 10:30:13PM +0800, Xu, Like wrote:
>
> > > Are you sure? Spurious NMI/PMIs are known to happen anyway. We have far
> > > too much code to deal with them.
> > https://lore.kernel.org/lkml/20170628130748.GI5981@leverpostej/T/
> >
> > In the rr workload, the commit change "the PMI interrupts in skid region
> > should be dropped"
> > is reverted since some users complain that:
> >
> > > It seems to me that it might be reasonable to ignore the interrupt if
> > > the purpose of the interrupt is to trigger sampling of the CPUs
> > > register state.  But if the interrupt will trigger some other
> > > operation, such as a signal on an fd, then there's no reason to drop
> > > it.
> > I assume that if the PMI drop is unacceptable, either will spurious PMI
> > injection.
> >
> > I'm pretty open if you insist that we really need to do this for guest PEBS
> > enabling.
> That was an entirely different issue. We were dropping events on the
> floor because they'd passed priv boundaries. So there was an actual
> event, and we made it go away.

Thanks for your clarification and support.

> What we're talking about here is raising an PMI with BUFFER_OVF set,
> even if the DS is empty. That should really be harmless. We'll take the
> PMI, find there's nothing there, and do nothing.

The only harm point is confusing the guest PEBS user with
the behavior of pebs_interrupt_threshold.

Now that KVM has to break it due to cross-mapping issue,
Let me implement this idea in the next version w/ relevant performance data.