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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     kvm@vger.kernel.org,
        Anirudh Rayabharam <anrayabh@linux.microsoft.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Maxim Levitsky <mlevitsk@redhat.com>,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 01/25] KVM: x86: hyper-v: Expose access to debug MSRs
 in the partition privilege flags
In-Reply-To: <0741719d-17b4-96fe-1ee8-5f22cf3e255b@redhat.com>
References: <20220714091327.1085353-1-vkuznets@redhat.com>
 <20220714091327.1085353-2-vkuznets@redhat.com>
 <YtnIgQOPbcZOQK2D@google.com>
 <0741719d-17b4-96fe-1ee8-5f22cf3e255b@redhat.com>
Date:   Mon, 01 Aug 2022 10:16:52 +0200
Message-ID: <87a68o2xaj.fsf@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 7/21/22 23:43, Sean Christopherson wrote:
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index c284a605e453..ca91547034e4 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -1282,7 +1282,7 @@ static bool hv_check_msr_access(struct kvm_vcpu_hv *hv_vcpu, u32 msr)
>>          case HV_X64_MSR_SYNDBG_OPTIONS:
>>          case HV_X64_MSR_SYNDBG_CONTROL ... HV_X64_MSR_SYNDBG_PENDING_BUFFER:
>>                  return hv_vcpu->cpuid_cache.features_edx &
>> -                       HV_FEATURE_DEBUG_MSRS_AVAILABLE;
>> +                       HV_ACCESS_DEBUG_MSRS;
>>          default:
>>                  break;
>>          }
>> 
>
> Yes, and this will need some kind of hack in QEMU to expose both CPUID 
> bits.  Fortunately hv-syndbg shouldn't be in much use in the wild, so I 
> think we can avoid quirks etc.

Properly behaving VMM should always expose both bits. I'm not sure what
would it mean if only 'access' bit is present: you can try accessing the
missing feature but you get #GP anyway most likely. When the feature is
available but 'access' bit is not set -- the result is also #GP. In case
we really want to support this behavior in KVM we should probably check
*both* bits in hv_check_msr_access() but I don't really see a
use-case. I've lazily kept HV_FEATURE_DEBUG_MSRS_AVAILABLE here just to
be QEMU compatible.

-- 
Vitaly