linux-kernel - Re: [PATCH RESEND v12 00/17] KVM: x86/pmu: Add basic support to enable guest PEBS via DS

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Message-ID: <874k1ltw9y.fsf@redhat.com>
Date:   Thu, 19 May 2022 16:46:49 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH RESEND v12 00/17] KVM: x86/pmu: Add basic support to
 enable guest PEBS via DS

Like Xu <like.xu.linux@...il.com> writes:

> On 19/5/2022 9:31 pm, Like Xu wrote:
>> ==== Test Assertion Failure ====
>>     lib/x86_64/processor.c:1207: r == nmsrs
>>     pid=6702 tid=6702 errno=7 - Argument list too long
>>        1    0x000000000040da11: vcpu_save_state at processor.c:1207 
>> (discriminator 4)
>>        2    0x00000000004024e5: main at state_test.c:209 (discriminator 6)
>>        3    0x00007f9f48c2d55f: ?? ??:0
>>        4    0x00007f9f48c2d60b: ?? ??:0
>>        5    0x00000000004026d4: _start at ??:?
>>     Unexpected result from KVM_GET_MSRS, r: 29 (failed MSR was 0x3f1)
>> 
>> I don't think any of these failing tests care about MSR_IA32_PEBS_ENABLE
>> in particular, they're just trying to do KVM_GET_MSRS/KVM_SET_MSRS.
>
> One of the lessons I learned here is that the members of msrs_to_save_all[]
> are part of the KVM ABI. We don't add feature-related MSRs until the last
> step of the KVM exposure feature (in this case, adding MSR_IA32_PEBS_ENABLE,
> MSR_IA32_DS_AREA, MSR_PEBS_DATA_CFG to msrs_to_save_all[] should take
> effect along with exposing the CPUID bits).

AFAIR the basic rule here is that whatever gets returned with
KVM_GET_MSR_INDEX_LIST can be passed to KVM_GET_MSRS and read
successfully by the host (not necessarily by the guest) so my guess is
that MSR_IA32_PEBS_ENABLE is now returned in KVM_GET_MSR_INDEX_LIST but
can't be read with KVM_GET_MSRS. Later, the expectation is that what was
returned by KVM_GET_MSRS can be set successfully with KVM_SET_MSRS.

-- 
Vitaly