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

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Message-ID: <e1dd4d82-b5d8-fdae-325b-75ba690eaf2e@gmail.com>
Date:   Fri, 4 Mar 2022 17:33:47 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Ravi Bangoria <ravi.bangoria@....com>
Cc:     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 4/3/2022 2:05 am, Jim Mattson wrote:
> 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?

The TSX story has more twists and turns, but we may start with 3a632cb229bf.

> 
>>>
>>> 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.

I could take over 3nd patch w/ Co-developed-by and move on if Ravi agrees.

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