Re: [PATCH v3 06/17] KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS

[Xu, Like]

On 2021/1/15 22:46, Peter Zijlstra wrote:
> On Mon, Jan 04, 2021 at 09:15:31PM +0800, Like Xu wrote:
>
> > +	if (cpuc->pebs_enabled & ~cpuc->intel_ctrl_host_mask) {
> > +		arr[1].msr = MSR_IA32_PEBS_ENABLE;
> > +		arr[1].host = cpuc->pebs_enabled & ~cpuc->intel_ctrl_guest_mask;
> > +		arr[1].guest = cpuc->pebs_enabled & ~cpuc->intel_ctrl_host_mask;
> > +		/*
> > +		 * The guest PEBS will be disabled once the host PEBS is enabled
> > +		 * since the both enabled case may brings a unknown PMI to
> > +		 * confuse host and the guest PEBS overflow PMI would be missed.
> > +		 */
> > +		if (arr[1].host)
> > +			arr[1].guest = 0;
> > +		arr[0].guest |= arr[1].guest;
> > +		*nr = 2;
> Elsewhere you write:
>
> > When we have a PEBS PMI due to guest workload and vm-exits,
> > the code path from vm-exit to the host PEBS PMI handler may also
> > bring PEBS PMI and mark the status bit. The current PMI handler
> > can't distinguish them and would treat the later one as a suspicious
> > PMI and output a warning.
> So the reason isn't that spurious PMIs are tedious, but that the
> hardware is actually doing something weird.
>
> Or am I not reading things straight?

I think the PEBS facility works as expected because in the both enabled case,
the first PEBS PMI is generated from host counter 1 based on guest 
interrupt_threshold
and the later PEBS PMI could be generated from host counter 2 based on host 
interrupt_threshold.

Therefore, if we adjust the overflow value to a small value, so that the 
number of
instructions from vm-exit to global ctrl disabling could be enough big to 
trigger PEBS PMI.

Do you think this is weird, or do you see other possibilities ?