linux-kernel - Re: [PATCH v4 08/12] KVM: x86/pmu: Disable vPMU if the minimum num of counters isn't met

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Date:   Thu, 6 Apr 2023 17:23:42 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 08/12] KVM: x86/pmu: Disable vPMU if the minimum num of
 counters isn't met

On Thu, Apr 06, 2023, Sean Christopherson wrote:
> On Tue, Feb 14, 2023, Like Xu wrote:
> > Suggested-by: Sean Christopherson <seanjc@...gle.com>
> > Signed-off-by: Like Xu <likexu@...cent.com>
> > ---
> >  arch/x86/kvm/pmu.h | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> > index d1cc02c8da88..46db5404894e 100644
> > --- a/arch/x86/kvm/pmu.h
> > +++ b/arch/x86/kvm/pmu.h
> > @@ -170,6 +170,12 @@ static inline void kvm_init_pmu_capability(const struct kvm_pmu_ops *pmu_ops)
> >  	if ((is_intel && !kvm_pmu_cap.version) || !kvm_pmu_cap.num_counters_gp)
> >  		enable_pmu = false;
> >  
> > +	/*
> > +	 * For AMD, disable vPMU if the minimum number of counters isn't met.
> > +	 */
> 
> Doesn't need to be a multiple line comment.  This comment is also useless.  It's
> quite clear from the code that PMU support is being disabled when there aren't
> enough counters, what's missing is _why_.
> 
> > +	if (!is_intel && kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS)

Actually, rather than bleed AMD details into common code, define a const int
kvm_pmu_ops, e.g.

	const int MIN_NR_GP_COUNTERS

and then the above !kvm_pmu_cap.num_counters_gp can get replaced with a more generic
check.   That will also give us a convenient location to document _why_ Intel
and AMD have different mins (in particular, AMD's lack of any way to enumerate
less than four counters to the guest).

E.g. end up with

	if (enable_pmu) {
		perf_get_x86_pmu_capability(&kvm_pmu_cap);

		/*
		 * For Intel, only support guest architectural pmu
		 * on a host with architectural pmu.
		 */
		if ((is_intel && !kvm_pmu_cap.version) ||
		    (kvm_pmu_cap.num_counters_gp < pmu_ops->MIN_NR_GP_COUNTERS))
			enable_pmu = false;
	}

Hmm, and doesn't have to be done in this series, but we could do the same for the
min/max PMU versions.

> > +		enable_pmu = false;
> > +
> >  	if (!enable_pmu) {
> >  		memset(&kvm_pmu_cap, 0, sizeof(kvm_pmu_cap));
> >  		return;
> > -- 
> > 2.39.1
> >