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

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Date:   Wed, 9 Mar 2022 19:47:12 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        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,
        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 3/9/22 19:35, Jim Mattson wrote:
> 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?
> 

I would expect that the same would happen on an SMI or a host interrupt.

	1:
	pause
	outl al, 0xb2
	jmp 1

In general a PAUSE vmexit will mostly benefit the VM that is pausing, so 
having a partial implementation would be better than disabling it 
altogether.

Paolo