linux-kernel - Re: [PATCH v6 04/16] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE

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Date:   Tue, 18 May 2021 15:49:37 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Venkatesh Srinivas <venkateshs@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, weijiang.yang@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>, ak@...ux.intel.com,
        wei.w.wang@...el.com, eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Yao Yuan <yuan.yao@...el.com>,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v6 04/16] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit
 when vPMU is enabled

On 2021/5/18 7:51, Sean Christopherson wrote:
> On Mon, May 17, 2021, Sean Christopherson wrote:
>> On Thu, May 13, 2021, Xu, Like wrote:
>>> On 2021/5/12 23:18, Sean Christopherson wrote:
>>>> On Wed, May 12, 2021, Xu, Like wrote:
>>>>> Hi Venkatesh Srinivas,
>>>>>
>>>>> On 2021/5/12 9:58, Venkatesh Srinivas wrote:
>>>>>> On 5/10/21, Like Xu <like.xu@...ux.intel.com> wrote:
>>>>>>> On Intel platforms, the software can use the IA32_MISC_ENABLE[7] bit to
>>>>>>> detect whether the processor supports performance monitoring facility.
>>>>>>>
>>>>>>> It depends on the PMU is enabled for the guest, and a software write
>>>>>>> operation to this available bit will be ignored.
>>>>>> Is the behavior that writes to IA32_MISC_ENABLE[7] are ignored (rather than #GP)
>>>>>> documented someplace?
>>>>> The bit[7] behavior of the real hardware on the native host is quite
>>>>> suspicious.
>>>> Ugh.  Can you file an SDM bug to get the wording and accessibility updated?  The
>>>> current phrasing is a mess:
>>>>
>>>>     Performance Monitoring Available (R)
>>>>     1 = Performance monitoring enabled.
>>>>     0 = Performance monitoring disabled.
>>>>
>>>> The (R) is ambiguous because most other entries that are read-only use (RO), and
>>>> the "enabled vs. disabled" implies the bit is writable and really does control
>>>> the PMU.  But on my Haswell system, it's read-only.
>>> On your Haswell system, does it cause #GP or just silent if you change this
>>> bit ?
>> Attempting to clear the bit generates a #GP.
> *sigh*
>
> Venkatesh and I are exhausting our brown paper bag supply.
>
> Attempting to clear bit 7 is ignored on both Haswell and Goldmont.  This _no_ #GP,
> the toggle is simply ignored.  I forgot to specify hex format (multiple times),
> and Venkatesh accessed the wrong MSR (0x10a instead of 0x1a0).

*sigh*

>
> So your proposal to ignore the toggle in KVM is the way to go, but please
> document in the changelog that that behavior matches bare metal.

Thank you, I will clearly state it in the commit message.

>
> It would be nice to get the SDM cleaned up to use "supported/unsupported", and to
> pick one of (R), (RO), and (R/O) for all MSRs entries for consistency, but that
> may be a pipe dream.

Glad you could review my code. I have reported this issue internally.

>
> Sorry for the run-around :-/