linux-kernel - Re: [PATCH] KVM: x86/pmu: omit "impossible" Intel counter MSRs from MSR list

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Date:   Wed, 7 Sep 2022 11:25:14 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jamttson@...gle.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/pmu: omit "impossible" Intel counter MSRs from
 MSR list

On 7/9/2022 8:37 am, Jim Mattson wrote:
> On Tue, Sep 6, 2022 at 1:16 AM Like Xu <like.xu.linux@...il.com> wrote:
>>
>> From: Like Xu <likexu@...cent.com>
>>
>> According to Intel April 2022 SDM - Table 2-2. IA-32 Architectural MSRs,
>> combined with the address reservation ranges of PERFCTRx, EVENTSELy, and
>> MSR_IA32_PMCz, the theoretical effective maximum value of the Intel GP
>> counters is 14, instead of 18:
>>
>>    14 = 0xE = min (
>>      0xE = IA32_CORE_CAPABILITIES (0xCF) - IA32_PMC0 (0xC1),
>>      0xF = IA32_OVERCLOCKING_STATUS (0x195) - IA32_PERFEVTSEL0 (0x186),
>>      0xF = IA32_MCG_EXT_CTL (0x4D0) - IA32_A_PMC0 (0x4C1)
>>    )
>>
>> the source of the incorrect number may be:
>>    18 = 0x12 = IA32_PERF_STATUS (0x198) - IA32_PERFEVTSEL0 (0x186)
>> but the range covers IA32_OVERCLOCKING_STATUS, which is also architectural.
>> Cut the list to 14 entries to avoid false positives.
>>
>> Cc: Kan Liang <kan.liang@...ux.intel.com>
>> Cc: Jim Mattson <jamttson@...gle.com>
> 
> That should be 'jmattson.'

Oops, my fault.

> 
>> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
>> Fixes: cf05a67b68b8 ("KVM: x86: omit "impossible" pmu MSRs from MSR list")
> 
> I'm not sure I completely agree with the "Fixes," since
> IA32_OVERCLOCKING_STATUS didn't exist back then. However, Paolo did
> make the incorrect assumption that Intel wouldn't cut the range even
> further with the introduction of new MSRs.

This new msr is added in April 2022.

Driver-like software had to keep up with real hardware changes and
speculatively with potential predictable hardware changes until failure.

> 
> To that point, aren't you setting yourself up for a future "Fixes"
> referencing this change?

(1) We have precedents like be4f3b3f8227;
(2) Fixes tags is introduced to help stable trees' maintainers (and their robot 
selectors)
absorb suitable patches like this one. We can expect similar issues with stable 
trees running
on new hardware without this fix.
(3) Fixing the tags does not feather the developer's nest, on the contrary the 
upstream code
itself as a vehicle for our group knowledge, is reinforced.

> 
> We should probably stop at the maximum number of GP PMCs supported
> today (8, I think).

I actually thought that at first, until I saw the speculative offset +17 :D.

> 
> If Intel doubles the number of PMCs to remain competitive with AMD,
> they'll probably put PMCs 8-15 in a completely different range of MSR
> indices.

I'll do a little cleanup work as the next version, stopping the number at 8.

> 
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> ---
>>   arch/x86/kvm/x86.c | 8 ++------
>>   1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 43a6a7efc6ec..98cdd4221447 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1431,8 +1431,6 @@ static const u32 msrs_to_save_all[] = {
>>          MSR_ARCH_PERFMON_PERFCTR0 + 8, MSR_ARCH_PERFMON_PERFCTR0 + 9,
>>          MSR_ARCH_PERFMON_PERFCTR0 + 10, MSR_ARCH_PERFMON_PERFCTR0 + 11,
>>          MSR_ARCH_PERFMON_PERFCTR0 + 12, MSR_ARCH_PERFMON_PERFCTR0 + 13,
>> -       MSR_ARCH_PERFMON_PERFCTR0 + 14, MSR_ARCH_PERFMON_PERFCTR0 + 15,
>> -       MSR_ARCH_PERFMON_PERFCTR0 + 16, MSR_ARCH_PERFMON_PERFCTR0 + 17,
>>          MSR_ARCH_PERFMON_EVENTSEL0, MSR_ARCH_PERFMON_EVENTSEL1,
>>          MSR_ARCH_PERFMON_EVENTSEL0 + 2, MSR_ARCH_PERFMON_EVENTSEL0 + 3,
>>          MSR_ARCH_PERFMON_EVENTSEL0 + 4, MSR_ARCH_PERFMON_EVENTSEL0 + 5,
>> @@ -1440,8 +1438,6 @@ static const u32 msrs_to_save_all[] = {
>>          MSR_ARCH_PERFMON_EVENTSEL0 + 8, MSR_ARCH_PERFMON_EVENTSEL0 + 9,
>>          MSR_ARCH_PERFMON_EVENTSEL0 + 10, MSR_ARCH_PERFMON_EVENTSEL0 + 11,
>>          MSR_ARCH_PERFMON_EVENTSEL0 + 12, MSR_ARCH_PERFMON_EVENTSEL0 + 13,
>> -       MSR_ARCH_PERFMON_EVENTSEL0 + 14, MSR_ARCH_PERFMON_EVENTSEL0 + 15,
>> -       MSR_ARCH_PERFMON_EVENTSEL0 + 16, MSR_ARCH_PERFMON_EVENTSEL0 + 17,
>>          MSR_IA32_PEBS_ENABLE, MSR_IA32_DS_AREA, MSR_PEBS_DATA_CFG,
>>
>>          MSR_K7_EVNTSEL0, MSR_K7_EVNTSEL1, MSR_K7_EVNTSEL2, MSR_K7_EVNTSEL3,
>> @@ -6943,12 +6939,12 @@ static void kvm_init_msr_list(void)
>>                                  intel_pt_validate_hw_cap(PT_CAP_num_address_ranges) * 2)
>>                                  continue;
>>                          break;
>> -               case MSR_ARCH_PERFMON_PERFCTR0 ... MSR_ARCH_PERFMON_PERFCTR0 + 17:
>> +               case MSR_ARCH_PERFMON_PERFCTR0 ... MSR_ARCH_PERFMON_PERFCTR0 + 13:
>>                          if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_PERFCTR0 >=
>>                              min(INTEL_PMC_MAX_GENERIC, kvm_pmu_cap.num_counters_gp))
>>                                  continue;
>>                          break;
>> -               case MSR_ARCH_PERFMON_EVENTSEL0 ... MSR_ARCH_PERFMON_EVENTSEL0 + 17:
>> +               case MSR_ARCH_PERFMON_EVENTSEL0 ... MSR_ARCH_PERFMON_EVENTSEL0 + 13:
>>                          if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >=
>>                              min(INTEL_PMC_MAX_GENERIC, kvm_pmu_cap.num_counters_gp))
>>                                  continue;
>> --
>> 2.37.3
>>