linux-kernel - Re: [PATCH 3/3] KVM: x86/pmu: Segregate Intel and AMD specific logic

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Date:   Thu, 3 Mar 2022 10:05:11 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     Like Xu <like.xu.linux@...il.com>, seanjc@...gle.com,
        dave.hansen@...ux.intel.com, peterz@...radead.org,
        alexander.shishkin@...ux.intel.com, eranian@...gle.com,
        daviddunn@...gle.com, ak@...ux.intel.com,
        kan.liang@...ux.intel.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, kim.phillips@....com,
        santosh.shukla@....com,
        "Paolo Bonzini - Distinguished Engineer (kernel-recipes.org) (KVM HoF)" 
        <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] KVM: x86/pmu: Segregate Intel and AMD specific logic

On Thu, Mar 3, 2022 at 8:25 AM Ravi Bangoria <ravi.bangoria@....com> wrote:
>
>
>
> On 03-Mar-22 10:08 AM, Jim Mattson wrote:
> > On Mon, Feb 21, 2022 at 2:02 AM Ravi Bangoria <ravi.bangoria@....com> wrote:
> >>
> >>
> >>
> >> On 21-Feb-22 1:27 PM, Like Xu wrote:
> >>> On 21/2/2022 3:31 pm, Ravi Bangoria wrote:
> >>>>   void reprogram_counter(struct kvm_pmu *pmu, int pmc_idx)
> >>>>   {
> >>>>       struct kvm_pmc *pmc = kvm_x86_ops.pmu_ops->pmc_idx_to_pmc(pmu, pmc_idx);
> >>>> +    bool is_intel = !strncmp(kvm_x86_ops.name, "kvm_intel", 9);
> >>>
> >>> How about using guest_cpuid_is_intel(vcpu)
> >>
> >> Yeah, that's better then strncmp().
> >>
> >>> directly in the reprogram_gp_counter() ?
> >>
> >> We need this flag in reprogram_fixed_counter() as well.
> >
> > Explicit "is_intel" checks in any form seem clumsy,
>
> Indeed. However introducing arch specific callback for such tiny
> logic seemed overkill to me. So thought to just do it this way.

I agree that arch-specific callbacks are ridiculous for these distinctions.

> > since we have
> > already put some effort into abstracting away the implementation
> > differences in struct kvm_pmu. It seems like these differences could
> > be handled by adding three masks to that structure: the "raw event
> > mask" (i.e. event selector and unit mask), the hsw_in_tx mask, and the
> > hsw_in_tx_checkpointed mask.
>
> struct kvm_pmu is arch independent. You mean struct kvm_pmu_ops?

No; I meant exactly what I said. See, for example, how the
reserved_bits field is used. It is initialized in the vendor-specific
pmu_refresh functions, and from then on, it facilitates
vendor-specific behaviors without explicit checks or vendor-specific
callbacks. An eventsel_mask field would be a natural addition to this
structure, to deal with the vendor-specific event selector widths. The
hsw_in_tx_mask and hsw_in_tx_checkpointed_mask fields are less
natural, since they will be 0 on AMD, but they would make it simple to
write the corresponding code in a vendor-neutral fashion.

BTW, am I the only one who finds the HSW_ prefixes a bit absurd here,
since TSX was never functional on Haswell?

> >
> > These changes should also be coordinated with Like's series that
> > eliminates all of the PERF_TYPE_HARDWARE nonsense.
>
> I'll rebase this on Like's patch series.

That's not exactly what I meant, but okay.