Re: [PATCH RFC v1 0/9] KVM: SVM: Defer page pinning for SEV guests

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 31, 2022, Peter Gonda wrote:
> On Wed, Mar 30, 2022 at 10:48 PM Nikunj A. Dadhania <nikunj@amd.com> wrote:
> > On 3/31/2022 1:17 AM, Sean Christopherson wrote:
> > > On Wed, Mar 30, 2022, Nikunj A. Dadhania wrote:
> > >> On 3/29/2022 2:30 AM, Sean Christopherson wrote:
> > >>> Let me preface this by saying I generally like the idea and especially the
> > >>> performance, but...
> > >>>
> > >>> I think we should abandon this approach in favor of committing all our resources
> > >>> to fd-based private memory[*], which (if done right) will provide on-demand pinning
> > >>> for "free".
> > >>
> > >> I will give this a try for SEV, was on my todo list.
> > >>
> > >>> I would much rather get that support merged sooner than later, and use
> > >>> it as a carrot for legacy SEV to get users to move over to its new APIs, with a long
> > >>> term goal of deprecating and disallowing SEV/SEV-ES guests without fd-based private
> > >>> memory.
> > >>
> > >>> That would require guest kernel support to communicate private vs. shared,
> > >>
> > >> Could you explain this in more detail? This is required for punching hole for shared pages?
> > >
> > > Unlike SEV-SNP, which enumerates private vs. shared in the error code, SEV and SEV-ES
> > > don't provide private vs. shared information to the host (KVM) on page fault.  And
> > > it's even more fundamental then that, as SEV/SEV-ES won't even fault if the guest
> > > accesses the "wrong" GPA variant, they'll silent consume/corrupt data.
> > >
> > > That means KVM can't support implicit conversions for SEV/SEV-ES, and so an explicit
> > > hypercall is mandatory.  SEV doesn't even have a vendor-agnostic guest/host paravirt
> > > ABI, and IIRC SEV-ES doesn't provide a conversion/map hypercall in the GHCB spec, so
> > > running a SEV/SEV-ES guest under UPM would require the guest firmware+kernel to be
> > > properly enlightened beyond what is required architecturally.
> > >
> >
> > So with guest supporting KVM_FEATURE_HC_MAP_GPA_RANGE and host (KVM) supporting
> > KVM_HC_MAP_GPA_RANGE hypercall, SEV/SEV-ES guest should communicate private/shared
> > pages to the hypervisor, this information can be used to mark page shared/private.
> 
> One concern here may be that the VMM doesn't know which guests have
> KVM_FEATURE_HC_MAP_GPA_RANGE support and which don't. Only once the
> guest boots does the guest tell KVM that it supports
> KVM_FEATURE_HC_MAP_GPA_RANGE. If the guest doesn't we need to pin all
> the memory before we run the guest to be safe to be safe.

Yep, that's a big reason why I view purging the existing SEV memory management as
a long term goal.  The other being that userspace obviously needs to be updated to
support UPM[*].   I suspect the only feasible way to enable this for SEV/SEV-ES
would be to restrict it to new VM types that have a disclaimer regarding additional
requirements.

[*] I believe Peter coined the UPM acronym for "Unmapping guest Private Memory".  We've
    been using it iternally for discussion and it rolls off the tongue a lot easier than
    the full phrase, and is much more precise/descriptive than just "private fd".