Re: [RFC PATCH 0/4] KVM: SEV: Limit cache flush operations in sev guest memory reclaim events

[Sean Christopherson <seanjc@google.com>]

On Fri, Dec 01, 2023, Mingwei Zhang wrote:
> On Fri, Dec 1, 2023 at 1:30 PM Kalra, Ashish <ashish.kalra@amd.com> wrote:
> > For SNP + gmem, where the HVA ranges covered by the MMU notifiers are
> > not acting on encrypted pages, we are ignoring MMU invalidation
> > notifiers for SNP guests as part of the SNP host patches being posted
> > upstream and instead relying on gmem own invalidation stuff to clean
> > them up on a per-folio basis.
> >
> > Thanks,
> > Ashish
> 
> oh, I have no question about that. This series only applies to
> SEV/SEV-ES type of VMs.
> 
> For SNP + guest_memfd, I don't see the implementation details, but I
> doubt you can ignore mmu_notifiers if the request does cover some
> encrypted memory in error cases or corner cases. Does the SNP enforce
> the usage of guest_memfd? How do we prevent exceptional cases? I am
> sure you guys already figured out the answers, so I don't plan to dig
> deeper until SNP host pages are accepted.

KVM will not allow SNP guests to map VMA-based memory as encrypted/private, full
stop.  Any invalidations initiated by mmu_notifiers will therefore apply only to
shared memory.

That approach doesn't work for SEV/SEV-ES because KVM can't prevent the guest
from accessing memory as encrypted, i.e. KVM needs the #NPF due to RMP violation
to intercept attempts to convert a GFN from shared to private.