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

[Marc Orr]

On Fri, Apr 1, 2022 at 11:02 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Apr 01, 2022, Marc Orr wrote:
> > On Thu, Mar 31, 2022 at 12:01 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > 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".
> >
> > Can we really "purge the existing SEV memory management"? This seems
> > like a non-starter because it violates userspace API (i.e., the
> > ability for the userspace VMM to run a guest without
> > KVM_FEATURE_HC_MAP_GPA_RANGE). Or maybe I'm not quite following what
> > you mean by purge.
>
> I really do mean purge, but I also really do mean "long term", as in 5+ years
> (probably 10+ if I'm being realistic).
>
> Removing support is completely ok, as is changing the uABI, the rule is that we
> can't break userspace.  If all users are migrated to private-fd, e.g. by carrots
> and/or sticks such as putting the code into maintenance-only mode, then at some
> point in the future there will be no users left to break and we can drop the
> current code and make use of private-fd mandatory for SEV/SEV-ES guests.

Ah, it makes sense now. Thanks!

> > Assuming that UPM-based lazy pinning comes together via a new VM type
> > that only supports new images based on a minimum kernel version with
> > KVM_FEATURE_HC_MAP_GPA_RANGE, then I think this would like as follows:
> >
> > 1. Userspace VMM: Check SEV VM type. If type is legacy SEV type then
> > do upfront pinning. Else, skip up front pinning.
>
> Yep, if by legacy "SEV type" you mean "SEV/SEV-ES guest that isn't required to
> use MAP_GPA_RANGE", which I'm pretty sure you do based on #3.

Yeah, that's exactly what I meant.

> > 2. KVM: I'm not sure anything special needs to happen here. For the
> > legacy VM types, it can be configured to use legacy memslots,
> > presumably the same as non-CVMs will be configured. For the new VM
> > type, it should be configured to use UPM.
>
> Correct, for now, KVM does nothing different for SEV/SEV-ES guests.
>
> > 3. Control plane (thing creating VMs): Responsible for not allowing
> > legacy SEV images (i.e., images without KVM_FEATURE_HC_MAP_GPA_RANGE)
> > with the new SEV VM types that use UPM and have support for demand
> > pinning.
> >
> > Sean: Did I get this right?
>
> Yep.

Thank you for verifying.