[PATCH 2/2] KVM: arm64: Commit pending PC adjustemnts before returning to userspace

[Fuad Tabba]

Hi Marc,


On Tue, May 11, 2021 at 9:14 AM Marc Zyngier <maz at kernel.org> wrote:
>
> Hi Fuad,
>
> On Tue, 11 May 2021 09:03:40 +0100,
> Fuad Tabba <tabba at google.com> wrote:
> >
> > Hi Marc,
> >
> > > KVM: arm64: Commit pending PC adjustemnts before returning to userspace
> >
> > s/adjustments/adjustments
>
> Looks like Gmail refuses to let you mimic my spelling mistakes! :D
>
> >
> > On Mon, May 10, 2021 at 10:49 AM Marc Zyngier <maz at kernel.org> wrote:
> > >
> > > KVM currently updates PC (and the corresponding exception state)
> > > using a two phase approach: first by setting a set of flags,
> > > then by converting these flags into a state update when the vcpu
> > > is about to enter the guest.
> > >
> > > However, this creates a disconnect with userspace if the vcpu thread
> > > returns there with any exception/PC flag set. In this case, the exposed
> > > context is wrong, as userpsace doesn't have access to these flags
> > > (they aren't architectural). It also means that these flags are
> > > preserved across a reset, which isn't expected.
> > >
> > > To solve this problem, force an explicit synchronisation of the
> > > exception state on vcpu exit to userspace. As an optimisation
> > > for nVHE systems, only perform this when there is something pending.
> >
> > I've tested this with a few nvhe and vhe tests that exercise both
> > __kvm_adjust_pc call paths (__kvm_vcpu_run and
> > kvm_arch_vcpu_ioctl_run), and the tests ran as expected.  I'll do the
> > same for v2 when you send it out.
>
> Ah, that's interesting. Do you have tests that actually fail when
> hitting this bug? Given that this is pretty subtle, it'd be good to
> have a way to make sure it doesn't crop up again.

Nothing that fails, just code that generates exceptions or emulates
instructions at various points. That said, I think it should be
straightforward to write a selftest for this. I'll give it a go.

/fuad

> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.