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References: <20220301143650.143749-1-mlevitsk@redhat.com> <20220301143650.143749-5-mlevitsk@redhat.com>
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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 9 Mar 2022 10:35:58 -0800
Message-ID: <CALMp9eRjY6sX0OEBeYw4RsQKSjKvXKWOqRe=GVoQnmjy6D8deg@mail.gmail.com>
Subject: Re: [PATCH v3 4/7] KVM: x86: nSVM: support PAUSE filter threshold and
 count when cpu_pm=on
To:     Maxim Levitsky <mlevitsk@redhat.com>
Cc:     kvm@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Joerg Roedel <joro@8bytes.org>, linux-kernel@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>
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On Tue, Mar 1, 2022 at 6:37 AM Maxim Levitsky <mlevitsk@redhat.com> wrote:
>
> Allow L1 to use these settings if L0 disables PAUSE interception
> (AKA cpu_pm=on)
>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

I didn't think pause filtering was virtualizable, since the value of
the internal counter isn't exposed on VM-exit.

On bare metal, for instance, assuming the hypervisor doesn't intercept
CPUID, the following code would quickly trigger a PAUSE #VMEXIT with
the filter count set to 2.

1:
pause
cpuid
jmp 1

Since L0 intercepts CPUID, however, L2 will exit to L0 on each loop
iteration, and when L0 resumes L2, the internal counter will be set to
2 again. L1 will never see a PAUSE #VMEXIT.

How do you handle this?