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

[Paolo Bonzini]

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