Re: Hyper-V VTLs, permission bitmaps and userspace exits (was Re: [PATCH 0/4] KVM: API to block and resume all running vcpus in a vm)

Sean Christopherson
2022-10-26T19:33:31Z

On Wed, Oct 26, 2022, Paolo Bonzini wrote:
> On 10/26/22 01:07, Sean Christopherson wrote:
> > > > - to stop anything else in the system that consumes KVM memslots, e.g. KVM GT
> > >
> > > Is this true if you only look at the KVM_GET_DIRTY_LOG case and consider it
> > > a guest bug to access the memory (i.e. ignore the strange read-only changes
> > > which only happen at boot, and which I agree are QEMU-specific)?
> >
> > Yes? I don't know exactly what "the KVM_GET_DIRTY_LOG case" is.
>
> It is not possible to atomically read the dirty bitmap and delete a memslot.
> When you delete a memslot, the bitmap is gone. In this case however memory
> accesses to the deleted memslot are a guest bug, so stopping KVM-GT would
> not be necessary.

If accesses to the deleted memslot are a guest bug, why do you care about pausing
vCPUs? I don't mean to be beligerent, I'm genuinely confused.

> So while I'm being slowly convinced that QEMU should find a way to pause its
> vCPUs around memslot changes, I'm not sure that pausing everything is needed
> in general.
>
> > > > And because of the nature of KVM, to support this API on all architectures, KVM
> > > > needs to make change on all architectures, whereas userspace should be able to
> > > > implement a generic solution.
> > >
> > > Yes, I agree that this is essentially just a more efficient kill().
> > > Emanuele, perhaps you can put together a patch to x86/vmexit.c in
> > > kvm-unit-tests, where CPU0 keeps changing memslots and the other CPUs are in
> > > a for(;;) busy wait, to measure the various ways to do it?
> >
> > I'm a bit confused. Is the goal of this to simplify QEMU, dedup VMM code, provide
> > a more performant solution, something else entirely?
>
> Well, a bit of all of them and perhaps that's the problem. And while the
> issues at hand *are* self-inflicted wounds on part of QEMU, it seems to me
> that the underlying issues are general.
>
> For example, Alex Graf and I looked back at your proposal of a userspace
> exit for "bad" accesses to memory, wondering if it could help with Hyper-V
> VTLs too. To recap, the "higher privileged" code at VTL1 can set up VM-wide
> restrictions on access to some pages through a hypercall
> (HvModifyVtlProtectionMask). After the hypercall, VTL0 would not be able to
> access those pages. The hypercall would be handled in userspace and would
> invoke a KVM_SET_MEMORY_REGION_PERM ioctl to restrict the RWX permissions,
> and this ioctl would set up a VM-wide permission bitmap that would be used
> when building page tables.
>
> Using such a bitmap instead of memslots makes it possible to cause userspace
> vmexits on VTL mapping violations with efficient data structures. And it
> would also be possible to use this mechanism around KVM_GET_DIRTY_LOG, to
> read the KVM dirty bitmap just before removing a memslot.

What exactly is the behavior you're trying to achieve for KVM_GET_DIRTY_LOG => delete?
If KVM provides KVM_EXIT_MEMORY_FAULT, can you not achieve the desired behavior by
doing mprotect(PROT_NONE) => KVM_GET_DIRTY_LOG => delete? If PROT_NONE causes the
memory to be freed, won't mprotect(PROT_READ) do what you want even without
KVM_EXIT_MEMORY_FAULT?

> However, external accesses to the regions (ITS, Xen, KVM-GT, non KVM_RUN
> ioctls) would not be blocked, due to the lack of a way to report the exit.

Aren't all of those out of scope? E.g. in a very hypothetical world where XEN's
event channel is being used with VTLs, if VTL1 makes the event channel inaccessible,
that's a guest and/or userspace configuration issue and the guest is hosed no matter
what KVM does. Ditto for these case where KVM-GT's buffer is blocked. I'm guessing
the ITS is similar?

> The intersection of these features with VTLs should be very small (sometimes
> zero since VTLs are x86 only), but the ioctls would be a problem so I'm
> wondering what your thoughts are on this.

How do the ioctls() map to VTLs? I.e. are they considered VTL0, VTL1, out-of-band?

> Also, while the exit API could be the same, it is not clear to me that the
> permission bitmap would be a good match for entirely "void" memslots used to
> work around non-atomic memslot changes. So for now let's leave this aside
> and only consider the KVM_GET_DIRTY_LOG case.

As above, can't userspace just mprotect() the entire memslot to prevent writes
between getting the dirty log and deleting the memslot?