Re: [PATCH 1/4] KVM: nSVM: Sync next_rip to cached vmcb12 after VMRUN of L2

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 23, 2026, Yosry Ahmed wrote:
> > > > > > Oh, good point.  In that case, I think it makes sense to add the flag asap, so
> > > > > > that _if_ it turns out that KVM needs to consume a field that isn't currently
> > > > > > saved/restored, we'll at least have a better story for KVM's that save/restore
> > > > > > everything.
> > > > >
> > > > > Not sure I follow. Do you mean start serializing everything and setting
> > > > > the flag ASAP (which IIUC would be after the rework we discussed),
> > > >
> > > > Yep.
> > >
> > > I don't think it matters that much when we start doing this. In all
> > > cases:
> > >
> > > 1. KVM will need to be backward-compatible.
> > >
> > > 2. Any new features that depend on save+restore of those fields will be
> > > a in a new KVM that does the 'full' save+restore (assuming we don't let
> > > people add per-field flags).
> > >
> > > The only scenario that I can think of is if a feature can be enabled at
> > > runtime, and we want to be able to enable it for a running VM after
> > > migrating from an old KVM to a new KVM. Not sure how likely this is.
> >
> > The scenario I'm thinking of is where we belatedly realize we should have been
> > saving+restoring a field for a feature that is already supported, e.g. gpat.  If
> > KVM saves+restores everything, then we don't have to come up with a hacky solution
> > for older KVM, because it already provides the desired behavior for the "save",
> > only the "restore" for the older KVM is broken.
> >
> > Does that make sense?  It makes sense in my head, but I'm not sure I communicated
> > the idea very well...
> 
> Kinda? What I am getting at is that we'll always have an old KVM that
> doesn't save everything that we'll need to handle. I think the
> scenario you have in mind is where we introduce a feature *after* we
> start saving everything, and at a later point realize we didn't add
> proper "restore" support, but the "save" support must have always been
> there.

Yes.

> gPAT is not a good example because it's in the "save" area :P

Oh, I don't think I realized this is only talking about the control area.  Or I
probably forgot.

> But yeah, I see your point. It's not very straightforward now because
> what we save comes from the cache, and we only cache what we need for
> the current set of features. So this will need to be done on top of
> the rework we've been discussing, where vmcb02 starts being the source
> of truth instead of the cache, then we just (mostly) save vmcb02's
> control area as-is.

Yeah, it's not urgent by any means.