Re: [PATCH v5 01/26] KVM: x86: hyper-v: Expose access to debug MSRs in the partition privilege flags

[Sean Christopherson]

On Tue, Aug 02, 2022, Vitaly Kuznetsov wrote:
> For some features, Hyper-V spec defines two separate CPUID bits: one
> listing whether the feature is supported or not and another one showing
> whether guest partition was granted access to the feature ("partition
> privilege mask"). 'Debug MSRs available' is one of such features. Add
> the missing 'access' bit.
> 
> Note: hv_check_msr_access() deliberately keeps checking
> HV_FEATURE_DEBUG_MSRS_AVAILABLE bit instead of the new HV_ACCESS_DEBUG_MSRS
> to not break existing VMMs (QEMU) which only expose one bit. Normally, VMMs
> should set either both these bits or none.

This is not the right approach long term.  If KVM absolutely cannot unconditionally
switch to checking HV_ACCESS_DEBUG_MSRS because it would break QEMU users, then we
should add a quirk, but sweeping the whole thing under the rug is wrong.