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Subject: Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in
 KVM_GET_SUPPORTED_CPUID
To:     Xiaoyao Li <xiaoyao.li@intel.com>,
        Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
References: <1603330475-7063-1-git-send-email-wanpengli@tencent.com>
 <cfd9d16f-6ddf-60d5-f73d-bb49ccd4055f@redhat.com>
 <6ad94df6-9ecd-e364-296a-34ba41e938b1@intel.com>
 <31b189e0-503f-157d-7af0-329744ed5369@redhat.com>
 <18e7a0c6-faff-8c4c-0830-a0bc02627a36@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <4a5d0ccd-f6a7-2afe-2480-991ff7e079b4@redhat.com>
Date:   Thu, 22 Oct 2020 16:44:52 +0200
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On 22/10/20 16:28, Xiaoyao Li wrote:
> On 10/22/2020 10:06 PM, Paolo Bonzini wrote:
>> On 22/10/20 15:31, Xiaoyao Li wrote:
>>>>
>>>> It's common for userspace to copy all supported CPUID bits to
>>>> KVM_SET_CPUID2, I don't think this is the right behavior for
>>>> KVM_HINTS_REALTIME.
>>>
>>> It reminds of X86_FEATURE_WAITPKG, which is added to supported CPUID
>>> recently as a fix but QEMU exposes it to guest only when "-overcommit
>>> cpu-pm"
>>
>> WAITPKG is not included in KVM_GET_SUPPORTED_CPUID either.  QEMU detects
>> it through the MSR_IA32_UMWAIT register.
> 
> Doesn't 0abcc8f65cc2 ("KVM: VMX: enable X86_FEATURE_WAITPKG in KVM
> capabilities") add WAITPKG to KVM_GET_SUPPORTED_CPUID?

You're right, I shouldn't have checked only cpuid.c. :)

Still I think WAITPKG is different, because it's only for userspace and
it's always possible for userspace to do "for(;;)" and burn CPU.
KVM_HINTS_REALTIME is more similar to MONITOR, which is not set.

Paolo