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

[Marc Zyngier]

Hi Oliver,

Adding Andrew and Peter to the discussion.

On Tue, 24 Aug 2021 22:15:03 +0100,
Oliver Upton <oupton at google.com> wrote:
> 
> Hey folks,
> 
> Ricardo and I were discussing hypercall support in KVM/arm64 and
> something seems to be a bit problematic. I do not see anywhere that KVM
> requires opt-in from the VMM to expose new hypercalls to the guest. To
> name a few, the TRNG and KVM PTP hypercalls are unconditionally provided
> to the guest.
> 
> Exposing new hypercalls to guests in this manner seems very unsafe to
> me. Suppose an operator is trying to upgrade from kernel N to kernel
> N+1, which brings in the new 'widget' hypercall. Guests are live
> migrated onto the N+1 kernel, but the operator finds a defect that
> warrants a kernel rollback. VMs are then migrated from kernel N+1 -> N.
> Any guests that discovered the 'widget' hypercall are likely going to
> get fussy _very_ quickly on the old kernel.

This goes against what we decided to support for the *only* publicly
available VMM that cares about save/restore, which is that we only
move forward and don't rollback. Hypercalls are the least of your
worries, and there is a whole range of other architectural features
that will have also appeared/disappeared (your own CNTPOFF series is a
glaring example of this).

I appreciate that you may have different considerations, but I felt
that it was important to state *why* this is the way it is.

> 
> Really, we need to ensure that the exposed guest ABI is
> backwards-compatible. Running a VMM that is blissfully unaware of the
> 'widget' hypercall should not implicitly expose it to its guest on a new
> kernel.
> 
> This conversation was in the context of devising a new UAPI that allows
> VMMs to trap hypercalls to userspace. While such an interface would
> easily work around the issue, the onus of ABI compatibility lies with
> the kernel.
> 
> So, this is all a long-winded way of asking: how do we dig ourselves out
> of this situation, and how to we avoid it happening again in the future?
> I believe the answer to both is to have new VM capabilities for sets of
> hypercalls exposed to the guest. Unfortunately, the unconditional
> exposure of TRNG and PTP hypercalls is ABI now, so we'd have to provide
> an opt-out at this point. For anything new, require opt-in from the VMM
> before we give it to the guest.
>
> Have I missed something blatantly obvious, or do others see this as an
> issue as well? I'll reply with an example of adding opt-out for PTP.
> I'm sure other hypercalls could be handled similarly.

Why do we need this? For future hypercalls, we could have some buy-in
capabilities. For existing ones, it is too late, and negative features
are just too horrible.

For KVM-specific hypercalls, we could get the VMM to save/restore the
bitmap of supported functions. That would be "less horrible". This
could be implemented using extra "firmware pseudo-registers" such as
the ones described in Documentation/virt/kvm/arm/psci.rst.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.