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

[Like Xu]

On 19/5/2022 10:46 pm, Vitaly Kuznetsov wrote:
> Like Xu <like.xu.linux@xxxxxxxxx> 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