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: <f379a933-15b0-6858-eeef-5fbef6e5529c@gmail.com>
Date:   Wed, 25 May 2022 15:56:42 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     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>
Subject: Re: [PATCH RESEND v12 00/17] KVM: x86/pmu: Add basic support to
 enable guest PEBS via DS

On 19/5/2022 10:46 pm, Vitaly Kuznetsov wrote:
> 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.
> 

Thanks for the clarification.

Some kvm x86 selftests have been failing due to this issue even after the last 
commit.

I blame myself for not passing the msr_info->host_initiated to the 
intel_is_valid_msr(),
meanwhile I pondered further whether we should check only the MSR addrs range in
the kvm_pmu_is_valid_msr() and apply this kind of sanity check in the 
pmu_set/get_msr().

Vitaly && Paolo, any preference to move forward ?

Thanks,
Like Xu