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 2:13 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > 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.
> 
> Remind me. If I (as a SEV-SNP guest) flip the C-bit in my own x86 page
> table and write to some of the pages, am I generating encrypted dirty
> cache lines?

No.  See Table 15-39. "RMP Memory Access Checks" in the APM (my attempts to copy
it to plain test failed miserably).  

For accesses with effective C-bit == 0, the page must be Hypervisor-Owned.  For
effective C-bit == 1, the page must be fully assigned to the guest.  Violation
of those rules generates #VMEXIT.

A missing Validated attribute causes a #VC, but that case has lower priority than
the about checks.