Re: [PATCH v3 4/7] KVM: x86: nSVM: support PAUSE filter threshold and count when cpu_pm=on

From: Paolo Bonzini <pbonzini@redhat.com>
To: Jim Mattson <jmattson@google.com>, 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, Wanpeng Li <wanpengli@tencent.com>
Subject: Re: [PATCH v3 4/7] KVM: x86: nSVM: support PAUSE filter threshold and count when cpu_pm=on
Date: Wed, 9 Mar 2022 19:47:12 +0100	[thread overview]
Message-ID: <6a7f13d1-ed00-b4a6-c39b-dd8ba189d639@redhat.com> (raw)
In-Reply-To: <CALMp9eRjY6sX0OEBeYw4RsQKSjKvXKWOqRe=GVoQnmjy6D8deg@mail.gmail.com>

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