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

[Jim Mattson <jmattson@google.com>]

On Wed, Mar 9, 2022 at 10:47 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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.

Indeed, the APM does say, "Certain events, including SMI, can cause
the internal count to be reloaded from the VMCB." However, expanding
that set of events so much that some pause loops will *never* trigger
a #VMEXIT seems problematic. If the hypervisor knew that the PAUSE
filter may not be triggered, it could always choose to exit on every
PAUSE.

Having a partial implementation is only better than disabling it
altogether if the L2 pause loop doesn't contain a hidden #VMEXIT to
L0.