Re: [PATCH 4/4] x86: kvm: mmu: use ept a/d in vmcs02 iff used in vmcs12

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/07/2017 09:29, Peter Feiner wrote:
> You're right: this is busted. I wrote these patches before you
> implemented EPT A/D nesting (i.e., PML was moot for guest mode).
> 
> I think the patch hunk can go away entirely actually. As long as PML
> is enabled, it's ok to flush the buffer. The interesting case is when
> the vcpu is in guest mode with EPT A/D disabled. In this case, L0's
> PML isn't filled while L2 runs because EPT A/D is disabled in the
> vmcs02 (thanks to this patch), so there's nothing in the buffer!

That was my thought too.

> It's troubling is that there's no test case covering L0's use of PML +
> nesting. Stress testing live migrations of L1 hypervisors (and
> implicitly their L2 guests) is one way of doing it, but it's pretty
> clumsy. A tightly coupled L0 userspace, L1 and L2 guests would be the
> way to go because you could just coordinate ioctls with guest memory
> accesses.

Indeed.  We need to do api/-style testing of nested virt.

Paolo