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From:   Reiji Watanabe <reijiw@google.com>
Date:   Mon, 26 Apr 2021 21:42:51 -0700
Message-ID: <CAAeT=FzVDFVCjYAZyc+QXwtLeOW5UR6AsYZwNT6kFbOwnn=xFQ@mail.gmail.com>
Subject: Re: [PATCH v3 3/4] KVM: x86: Tie Intel and AMD behavior for
 MSR_TSC_AUX to guest CPU model
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
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> > It looks Table 2-2 of the Intel SDM Vol4 (April 2021) says
> > TSC_AUX is supported:
> >
> >    If CPUID.80000001H:EDX[27] = 1 or CPUID.(EAX=7,ECX=0):ECX[22] = 1
> >
> > Should we also check X86_FEATURE_RDPID before returning 1
> > due to no RDTSCP support ?
>
> Yep.  VMX should also clear RDPID if the ENABLE_RDTSCP control isn't supported.
> That bug isn't fatal because KVM emulates RDPID on #UD, but it would be a
> notieable performance hit for the guest.

Thank you so much for the confirmation and the information.
Understood.


> There is also a kernel bug lurking; vgetcpu_cpu_init() doesn't check
> X86_FEATURE_RDPID and will fail to initialize MSR_TSC_AUX if RDPID is supported
> but RDTSCP is not, and __getcpu() uses RDPID.  I'll verify that's broken and
> send a patch for that one too.

I don't find vgetcpu_cpu_init() or __getcpu() in
https://github.com/torvalds/linux.
I would assume you meant setup_getcpu() and vdso_read_cpunode() instead (?).


> AMD also documents this in Appendix E:
>
>   CPUID Fn0000_0007_EBX_x0 Structured Extended Feature Identifiers (ECX=0)
>   Bits   Field   Name
>   ...
>   22     RDPID   RDPID instruction and TSC_AUX MSR support.

Thank you.  I overlooked that...


Regards,
Reiji