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

[Jim Mattson <jmattson@google.com>]

On Mon, Mar 21, 2022 at 2:36 PM Maxim Levitsky <mlevitsk@redhat.com> wrote:
>
> On Wed, 2022-03-09 at 11:07 -0800, Jim Mattson wrote:
> > 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.
> >
>
> Hi!
>
> You bring up a very valid point, which I didn't think about.
>
> However after thinking about this, I think that in practice,
> this isn't a show stopper problem for exposing this feature to the guest.
>
>
> This is what I am thinking:
>
> First lets assume that the L2 is malicious. In this case no doubt
> it can craft such a loop which will not VMexit on PAUSE.
> But that isn't a problem - instead of this guest could have just used NOP
> which is not possible to intercept anyway - no harm is done.
>
> Now lets assume a non malicious L2:
>
>
> First of all the problem can only happen when a VM exit is intercepted by L0,
> and not by L1. Both above cases usually don't pass this criteria since L1 is highly
> likely to intercept both CPUID and IO port access. It is also highly unlikely
> to allow L2 direct access to L1's mmio ranges.
>
> Overall there are very few cases of deterministic vm exit which is intercepted
> by L0 but not L1. If that happens then L1 will not catch the PAUSE loop,
> which is not different much from not catching it because of not suitable
> thresholds.
>
> Also note that this is an optimization only - due to count and threshold,
> it is not guaranteed to catch all pause loops - in fact hypervisor has
> to guess these values, and update them in attempt to catch as many such
> loops as it can.
>
> I think overall it is OK to expose that feature to the guest
> and it should even improve performance in some cases - currently
> at least nested KVM intercepts every PAUSE otherwise.

Can I at least request that this behavior be documented as a KVM
virtual CPU erratum?

>
> Best regards,
>         Maxim Levitsky
>
>
>
>