Re: [PATCH v1 1/5] KVM: arm64: Enable ring-based dirty memory tracking

Peter Xu
2022-08-23T23:19:18Z

On Tue, Aug 23, 2022 at 11:47:03PM +0100, Marc Zyngier wrote:
> On Tue, 23 Aug 2022 22:20:32 +0100,
> Peter Xu <peterx@redhat.com> wrote:
> >
> > On Tue, Aug 23, 2022 at 08:17:03PM +0100, Marc Zyngier wrote:
> > > I don't think we really need this check on the hot path. All we need
> > > is to make the request sticky until userspace gets their act together
> > > and consumes elements in the ring. Something like:
> > >
> > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > index 986cee6fbc7f..e8ed5e1af159 100644
> > > --- a/arch/arm64/kvm/arm.c
> > > +++ b/arch/arm64/kvm/arm.c
> > > @@ -747,6 +747,14 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
> > >
> > > if (kvm_check_request(KVM_REQ_SUSPEND, vcpu))
> > > return kvm_vcpu_suspend(vcpu);
> > > +
> > > + if (kvm_check_request(KVM_REQ_RING_SOFT_FULL, vcpu) &&
> > > + kvm_dirty_ring_soft_full(vcpu)) {
> > > + kvm_make_request(KVM_REQ_RING_SOFT_FULL, vcpu);
> > > + vcpu->run->exit_reason = KVM_EXIT_DIRTY_RING_FULL;
> > > + trace_kvm_dirty_ring_exit(vcpu);
> > > + return 0;
> > > + }
> > > }
> > >
> > > return 1;
> >
> > Right, this seems working. We can also use kvm_test_request() here.
> >
> > >
> > >
> > > However, I'm a bit concerned by the reset side of things. It iterates
> > > over the vcpus and expects the view of each ring to be consistent,
> > > even if userspace is hacking at it from another CPU. For example, I
> > > can't see what guarantees that the kernel observes the writes from
> > > userspace in the order they are being performed (the documentation
> > > provides no requirements other than "it must collect the dirty GFNs in
> > > sequence", which doesn't mean much from an ordering perspective).
> > >
> > > I can see that working on a strongly ordered architecture, but on
> > > something as relaxed as ARM, the CPUs may^Wwill aggressively reorder
> > > stuff that isn't explicitly ordered. I have the feeling that a CAS
> > > operation on both sides would be enough, but someone who actually
> > > understands how this works should have a look...
> >
> > I definitely don't think I 100% understand all the ordering things since
> > they're complicated.. but my understanding is that the reset procedure
> > didn't need memory barrier (unlike pushing, where we have explicit wmb),
> > because we assumed the userapp is not hostile so logically it should only
> > modify the flags which is a 32bit field, assuming atomicity guaranteed.
>
> Atomicity doesn't guarantee ordering, unfortunately.

Right, sorry to be misleading. The "atomicity" part I was trying to say
the kernel will always see consistent update on the fields.

The ordering should also be guaranteed, because things must happen with
below sequence:

(1) kernel publish dirty GFN data (slot, offset)
(2) kernel publish dirty GFN flag (set to DIRTY)
(3) user sees DIRTY, collects (slots, offset)
(4) user sets it to RESET
(5) kernel reads RESET

So the ordering of single-entry is guaranteed in that when (5) happens it
must be after stablized (1+2).

> Take the
> following example: CPU0 is changing a bunch of flags for GFNs A, B, C,
> D that exist in the ring in that order, and CPU1 performs an ioctl to
> reset the page state.
>
> CPU0:
> write_flag(A, KVM_DIRTY_GFN_F_RESET)
> write_flag(B, KVM_DIRTY_GFN_F_RESET)
> write_flag(C, KVM_DIRTY_GFN_F_RESET)
> write_flag(D, KVM_DIRTY_GFN_F_RESET)
> [...]
>
> CPU1:
> ioctl(KVM_RESET_DIRTY_RINGS)
>
> Since CPU0 writes do not have any ordering, CPU1 can observe the
> writes in a sequence that have nothing to do with program order, and
> could for example observe that GFN A and D have been reset, but not B
> and C. This in turn breaks the logic in the reset code (B, C, and D
> don't get reset), despite userspace having followed the spec to the
> letter. If each was a store-release (which is the case on x86), it
> wouldn't be a problem, but nothing calls it in the documentation.
>
> Maybe that's not a big deal if it is expected that each CPU will issue
> a KVM_RESET_DIRTY_RINGS itself, ensuring that it observe its own
> writes. But expecting this to work across CPUs without any barrier is
> wishful thinking.

I see what you meant...

Firstly I'm actually curious whether that'll really happen if the gfns are
collected in something like a for loop:

for(i = 0; i < N; i++)
collect_dirty_gfn(ring, i);

Because since all the gfps to be read will depend on variable "i", IIUC no
reordering should happen, but I'm not really sure, so more of a pure
question.

Besides, the other thing to mention is that I think it is fine the RESET
ioctl didn't recycle all the gfns got set to reset state. Taking above
example of GFNs A-D, if when reaching the RESET ioctl only A & D's flags
are updated, the ioctl will recycle gfn A but stop at gfn B assuming B-D
are not reset. But IMHO it's okay because it means we reset partial of the
gfns not all of them, and it's safe to do so. It means the next ring full
event can come earlier because we recycled less, but that's functionally
safe to me.

>
> > IIRC we used to discuss similar questions on "what if the user is hostile
> > and wants to hack the process by messing up with the ring", and our
> > conclusion was as long as the process wouldn't mess up anything outside
> > itself it should be okay. E.g. It should not be able to either cause the
> > host to misfunction, or trigger kernel warnings in dmesg, etc..
>
> I'm not even discussing safety here. I'm purely discussing the
> interactions between userspace and kernel based on the documentation
> and the existing kernel code.

I see. Please let me know if above makes sense.

Thanks!

--
Peter Xu