linux-kernel - Re: [PATCH v3 4/7] KVM: x86: nSVM: support PAUSE filter threshold and count when cpu

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Message-ID: <CALMp9eRjY6sX0OEBeYw4RsQKSjKvXKWOqRe=GVoQnmjy6D8deg@mail.gmail.com>
Date:   Wed, 9 Mar 2022 10:35:58 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH v3 4/7] KVM: x86: nSVM: support PAUSE filter threshold and
 count when cpu_pm=on

On Tue, Mar 1, 2022 at 6:37 AM Maxim Levitsky <mlevitsk@...hat.com> wrote:
>
> Allow L1 to use these settings if L0 disables PAUSE interception
> (AKA cpu_pm=on)
>
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.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?