On Wed, 25 Aug 2021 11:02:28 +0100, Oliver Upton <oupton@xxxxxxxxxx> wrote: > > On Wed, Aug 25, 2021 at 2:27 AM Marc Zyngier <maz@xxxxxxxxxx> 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. M. -- Without deviation from the norm, progress is not possible.
