Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases

Edgecombe, Rick P
2024-03-27T17:36:14Z

On Wed, 2024-03-27 at 10:54 +0800, Xiaoyao Li wrote:
> > > If QEMU doesn't configure the msr filter list correctly, KVM has to handle
> > > guest's MTRR MSR accesses. In my understanding, the suggestion is KVM zap
> > > private memory mappings. But guests won't accept memory again because no one
> > > currently requests guests to do this after writes to MTRR MSRs. In this case,
> > > guests may access unaccepted memory, causing infinite EPT violation loop
> > > (assume SEPT_VE_DISABLE is set). This won't impact other guests/workloads on
> > > the host. But I think it would be better if we can avoid wasting CPU resource
> > > on the useless EPT violation loop.
> >
> > Qemu is expected to do it correctly.  There are manyways for userspace to go
> > wrong.  This isn't specific to MTRR MSR.
>
> This seems incorrect. KVM shouldn't force userspace to filter some
> specific MSRs. The semantic of MSR filter is userspace configures it on
> its own will, not KVM requires to do so.

I'm ok just always doing the exit to userspace on attempt to use MTRRs in a TD, and not rely on the
MSR list. At least I don't see the problem.