linux-kernel - Re: [PATCH V2 09/23] perf/x86/intel: Check Arch LBR MSRs

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Date:   Tue, 30 Jun 2020 11:29:30 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, acme@...nel.org, tglx@...utronix.de,
        bp@...en8.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, dave.hansen@...el.com,
        yu-cheng.yu@...el.com, bigeasy@...utronix.de, gorcunov@...il.com,
        hpa@...or.com, alexey.budankov@...ux.intel.com, eranian@...gle.com,
        ak@...ux.intel.com, like.xu@...ux.intel.com,
        yao.jin@...ux.intel.com, wei.w.wang@...el.com
Subject: Re: [PATCH V2 09/23] perf/x86/intel: Check Arch LBR MSRs



On 6/30/2020 10:57 AM, Peter Zijlstra wrote:
> On Fri, Jun 26, 2020 at 11:20:06AM -0700, kan.liang@...ux.intel.com wrote:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>>
>> The KVM may not support the MSRs of Architecture LBR. Accessing the
>> MSRs may cause #GP and crash the guest.
>>
>> The MSRs have to be checked at guest boot time.
>>
>> Only using the max number of Architecture LBR depth to check the
>> MSR_ARCH_LBR_DEPTH should be good enough. The max number can be
>> calculated by 8 * the position of the last set bit of LBR_DEPTH value
>> in CPUID enumeration.
> 
> But But But, this is architectural, it's in CPUID. If KVM lies to us, it
> gets to keep the pices.
> 
> This was different when it was not enumerated and all we had was poking
> the MSRs, but here KVM can simply mask the CPUID bits if it doesn't
> support the MSRs.
> 
> If KVM gives us the CPUID bits, we should let it crash and burn if it
> then doesn't provide the MSRs.
> 

Agree.
If the CPUID bits are not set by KVM, the x86_pmu.lbr_nr should be 0.
The check will be ignored.

I think we just need to simply drop this patch.


Thanks,
Kan