KVM/arm64: Guest ABI changes do not appear rollback-safe

[Oliver Upton]

On Thu, Aug 26, 2021 at 09:37:42AM +0100, Marc Zyngier wrote:
> On Wed, 25 Aug 2021 19:14:59 +0100,
> Oliver Upton <oupton at google.com> wrote:
> > 
> > On Wed, Aug 25, 2021 at 8:07 AM Andrew Jones <drjones at redhat.com> wrote:
> 
> [...]
> 
> > > Thanks for including me Marc. I think you've mentioned all the examples
> > > of why we don't generally expect N+1 -> N migrations to work that I
> > > can think of. While some of the examples like get-reg-list could
> > > eventually be eliminated if we had CPU models to tighten our machine type
> > > state, I think N+1 -> N migrations will always be best effort at most.
> > >
> > > I agree with giving userspace control over the exposer of the hypercalls
> > > though. Using pseudo-registers for that purpose rather than a pile of
> > > CAPs also seems reasonable to me.
> > >
> > > And, while I don't think this patch is going to proceed, I thought I'd
> > > point out that the opt-out approach doesn't help much with expanding
> > > our migration support unless we require the VMM to be upgraded first.
> > >
> > > And, even then, the (N_kern, N+1_vmm) -> (N+1_kern, N_vmm) case won't
> > > work as expected, since the source enforce opt-out, but the destination
> > > won't.
> > 
> > Right, there's going to need to be a fence in both kernel and VMM
> > versions. Before the fence, you can't rollback with either component.
> > Once on the other side of the fence, the user may freely migrate
> > between kernel + VMM combinations.
> >
> > > Also, since the VMM doesn't key off the kernel version, for the
> > > most part N+1 VMMs won't know when they're supposed to opt-out or not,
> > > leaving it to the user to ensure they consider everything. opt-in
> > > usually only needs the user to consider what machine type they want to
> > > launch.
> > 
> > Going the register route will implicitly require opt-out for all old
> > hypercalls. We exposed them unconditionally to the guest before, and
> > we must uphold that behavior. The default value for the bitmap will
> > have those features set. Any hypercalls added after that register
> > interface will then require explicit opt-in from userspace.
> 
> I disagree here. This makes the ABI inconsistent, and means that no
> feature can be implemented without changing userspace. If you can deal
> with the existing features, you should be able to deal with the next
> lot.
>
> > With regards to the pseudoregister interface, how would a VMM discover
> > new bits? From my perspective, you need to have two bitmaps that the
> > VMM can get at: the set of supported feature bits and the active
> > bitmap of features for a running guest.
> 
> My proposal is that we have a single pseudo-register exposing the list
> of implemented by the kernel. Clear the bits you don't want, and write
> back the result. As long as you haven't written anything, you have the
> full feature set. That's pretty similar to the virtio feature
> negotiation.

Ah, yes I agree. Thinking about it more we will not need something
similar to KVM_GET_SUPPORTED_CPUID.

So then, for any register where userspace/KVM need to negotiate
features, the default value will return the maximum feature set that is
supported. If userspace wants to constrain features, read out the
register, make sure everything you want is there, and write it back
blowing away the superfluous bits. Given this should we enforce ordering
on feature registers, such that a VMM can only write to the registers
before a VM is started?

Also, Reiji is working on making the identity registers writable for the
sake of feature restriction. The suggested negotiation interface would
be applicable there too, IMO.

Many thanks to both you and Drew for working this out with me.

--
Best,
Oliver