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

[Andrew Jones]

On Wed, Aug 25, 2021 at 11:39:34AM +0100, Marc Zyngier wrote:
> On Wed, 25 Aug 2021 11:02:28 +0100,
> Oliver Upton <oupton at google.com> wrote:
> > 
> > On Wed, Aug 25, 2021 at 2:27 AM Marc Zyngier <maz at kernel.org> wrote:
> > > > 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.
> > 
> > Ah, I was definitely missing this context. Current behavior makes much
> > more sense then.
> > 
> > > 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).
> > 
> > Isn't that a tad bit different though? I'll admit, I'm just as guilty
> > with my own series forgetting to add a KVM_CAP (oops), but it is in my
> > queue to kick out with the fix for nVHE/ptimer. Nonetheless, if a user
> > takes up a new KVM UAPI, it is up to the user to run on a new kernel.
> 
> The two are linked. Exposing a new register to userspace and/or guest
> result in the same thing: you can't rollback. That's specially true in
> the QEMU case, which *learns* from the kernel what registers are
> available, and doesn't maintain a fixed list.
> 
> > My concerns are explicitly with the 'under the nose' changes, where
> > KVM modifies the guest feature set without userspace opting in. Based
> > on your comment, though, it would appear that other parts of KVM are
> > affected too.
> 
> Any new system register that is exposed by a new kernel feature breaks
> rollback. And so far, we only consider it a bug if the set of exposed
> registers reduces. Anything can be added safely (as checked by one of
> the selftests added by Drew).
> 
> < It doesn't have to be rollback safety, either. There may
> > simply be a hypercall which an operator doesn't want to give its
> > guests, and it needs a way to tell KVM to hide it.
> 
> Fair enough. But this has to be done in a scalable way, which
> individual capability cannot provide.
> 
> > > > 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.
> > 
> > Oh, agreed on the nastiness. Lazy hack to realize the intended
> > functional change..
> 
> Well, you definitely achieved your goal of attracting my attention :).
> 
> > > 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.
> > 
> > This seems more reasonable, especially since we do this for migrating
> > the guest's PSCI version.
> > 
> > Alternatively, I had thought about using a VM attribute, given the
> > fact that it is non-architectural information and we avoid ABI issues
> > in KVM_GET_REG_LIST without buy-in through a KVM_CAP.
> 
> The whole point is that these settings get exposed by
> KVM_GET_REG_LIST, as this is QEMU's way to dump a VM state. Given that
> we already have this for things like the spectre management state, we
> can just as well expose the bitmaps that deal with the KVM-specific
> hypercalls. After all, this falls into the realm of "KVM as VM
> firmware".
> 
> For ARM-architected hypercalls (TRNG, stolen time), we may need a
> similar extension.
> 

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. 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.

Thanks,
drew