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From: Reiji Watanabe <reijiw@google.com>
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
Subject: Re: [PATCH v3 3/4] KVM: x86: Tie Intel and AMD behavior for MSR_TSC_AUX to guest CPU model
Date: Mon, 26 Apr 2021 21:42:51 -0700	[thread overview]
Message-ID: <CAAeT=FzVDFVCjYAZyc+QXwtLeOW5UR6AsYZwNT6kFbOwnn=xFQ@mail.gmail.com> (raw)
In-Reply-To: <YIcWvcneHWA9OPxv@google.com>

> > 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

  reply	other threads:[~2021-04-27  4:43 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-23 22:34 [PATCH v3 0/4] KVM: x86: MSR_TSC_AUX fixes and improvements Sean Christopherson
2021-04-23 22:34 ` [PATCH v3 1/4] KVM: SVM: Inject #GP on guest MSR_TSC_AUX accesses if RDTSCP unsupported Sean Christopherson
2021-04-26  8:49   ` Vitaly Kuznetsov
2021-04-23 22:34 ` [PATCH v3 2/4] KVM: SVM: Clear MSR_TSC_AUX[63:32] on write Sean Christopherson
2021-04-26  8:57   ` Vitaly Kuznetsov
2021-04-26 19:18     ` Sean Christopherson
2021-04-26 19:44       ` Sean Christopherson
2021-04-23 22:34 ` [PATCH v3 3/4] KVM: x86: Tie Intel and AMD behavior for MSR_TSC_AUX to guest CPU model Sean Christopherson
2021-04-24  7:19   ` Reiji Watanabe
2021-04-26 19:38     ` Sean Christopherson
2021-04-27  4:42       ` Reiji Watanabe [this message]
2021-04-27 21:58         ` Sean Christopherson
2021-04-23 22:34 ` [PATCH v3 4/4] KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace Sean Christopherson
2021-04-26  9:26 ` [PATCH v3 0/4] KVM: x86: MSR_TSC_AUX fixes and improvements Paolo Bonzini

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