Re: [PATCH] perf/core: fix the bug in the event multiplexing

[Oliver Upton]

Hi Huang,

On Wed, Aug 09, 2023 at 09:39:53AM +0800, Huang Shijie wrote:
> 2.) Root cause.
> 	There is only 7 counters in my arm64 platform:
> 	  (one cycle counter) + (6 normal counters)
> 
> 	In 1.3 above, we will use 10 event counters.
> 	Since we only have 7 counters, the perf core will trigger
>        	event multiplexing in hrtimer:
> 	     merge_sched_in() -->perf_mux_hrtimer_restart() -->
> 	     perf_rotate_context().
> 
>        In the perf_rotate_context(), it does not restore some PMU registers
>        as context_switch() does.  In context_switch():
>              kvm_sched_in()  --> kvm_vcpu_pmu_restore_guest()
>              kvm_sched_out() --> kvm_vcpu_pmu_restore_host()
> 
>        So we got wrong result.

This is a rather vague description of the problem. AFAICT, the
issue here is on VHE systems we wind up getting the EL0 count
enable/disable bits backwards when entering the guest, which is
corroborated by the data you have below.

> +void arch_perf_rotate_pmu_set(void)
> +{
> +	if (is_guest())
> +		kvm_vcpu_pmu_restore_guest(NULL);
> +	else
> +		kvm_vcpu_pmu_restore_host(NULL);
> +}
> +

This sort of hook is rather nasty, and I'd strongly prefer a solution
that's confined to KVM. I don't think the !is_guest() branch is
necessary at all. Regardless of how the pmu context is changed, we need
to go through vcpu_put() before getting back out to userspace.

We can check for a running vCPU (ick) from kvm_set_pmu_events() and either
do the EL0 bit flip there or make a request on the vCPU to call
kvm_vcpu_pmu_restore_guest() immediately before reentering the guest.
I'm slightly leaning towards the latter, unless anyone has a better idea
here.

-- 
Thanks,
Oliver