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

[Peter Zijlstra]

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?