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SubjectRe: [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to the guest
On 01/03/2019 11:25 PM, Jim Mattson wrote:
> On Wed, Jan 2, 2019 at 11:55 PM Wei Wang <wei.w.wang@intel.com> wrote:
>
>> Right, thanks. Probably better to change it to below:
>>
>> msr_info->data = 0;
>> data = native_read_msr(MSR_IA32_PERF_CAPABILITIES);
>> if (vcpu->kvm->arch.lbr_in_guest)
>> msr_info->data |= (data & X86_PERF_CAP_MASK_LBR_FMT);
>>
> This still breaks backwards compatibility. Returning 0 and raising #GP
> are not the same.

I'm not sure about raising GP# in this case.

This PERF_CAP msr contains more things than the lbr format.
For example, a guest with lbr=false option could read it to get PEBS_FMT,
which is PERF_CAP[11:8]. We should offer those bits in this case.

When lbr=false, the lbr feature is not usable by the guest,
so I think whatever value (0 or other value) of the LBR_FMT bits that
we give to the guest might not be important.

Best,
Wei

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