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

[Sean Christopherson]

On Tue, Feb 08, 2022, Oliver Upton wrote:
> Hi Marc,
> 
> On Tue, Feb 8, 2022 at 1:46 AM Marc Zyngier <maz at kernel.org> wrote:
> > > > KVM currently restricts the vcpu features to be unified across vcpus,
> > > > but that's only an implementation choice.
> > >
> > > But that implementation choice has become ABI, no? How could support
> > > for asymmetry be added without requiring userspace opt-in or breaking
> > > existing VMMs that depend on feature unification?
> >
> > Of course, you'd need some sort of advertising of this new behaviour.
> >
> > One thing I would like to add to the current state of thing is an
> > indication of whether the effects of a sysreg being written from
> > userspace are global or local to a vcpu. You'd need a new capability,
> > and an extra flag added to the encoding of each register.
> 
> Ah. I think that is a much more reasonable fit then. VMMs unaware of
> this can continue to migrate new bits (albeit at the cost of
> potentially higher lock contention for the per-VM stuff), and those
> that do can reap the benefits of writing such attributes exactly once.

But the "proper" usage is no different than adding support for VM-scoped variants
of KVM_{G,S}ET_ONE_REG and friends, and a VM-scoped variant is conceptually a lot
cleaner IMO.  And making them truly VM-scoped means KVM can do things like support
sysregs that are immutable after vCPUs are created.

So long as KVM defaults to '0' for all such registers, lack of migration support
in userspace that isn't aware of the new API, i.e. doesn't do KVM_GET_REG_LIST at
a VM-scope, is a nop because said userspace also won't modify the registers in the
first place.  The only "unsolvable" problem that is avoided by usurping the per-vCPU
ioctls is rollback to a userspace VMM that isn't aware of the per-VM ioctls, but it
doesn't seem too onerous to tell userspace "don't use these unless your entire fleet
has upgraded", especially since that requirement/advisement is true for the KVM side
with respect to new registers regardless of how those registers are accessed.