Re: [PATCH v4 1/2] target/arm: kvm: Handle DABT with no valid ISS

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Andrew Jones (drjones@redhat.com) wrote:
> On Fri, Apr 17, 2020 at 11:39:25AM +0100, Peter Maydell wrote:
> > On Mon, 23 Mar 2020 at 11:32, Beata Michalska
> > <beata.michalska@linaro.org> wrote:
> > >
> > > On ARMv7 & ARMv8 some load/store instructions might trigger a data abort
> > > exception with no valid ISS info to be decoded. The lack of decode info
> > > makes it at least tricky to emulate those instruction which is one of the
> > > (many) reasons why KVM will not even try to do so.
> > >
> > > Add support for handling those by requesting KVM to inject external
> > > dabt into the quest.
> > >
> > > Signed-off-by: Beata Michalska <beata.michalska@linaro.org>
> > > ---
> > >  target/arm/cpu.h     |  2 ++
> > >  target/arm/kvm.c     | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  target/arm/kvm_arm.h | 11 +++++++++++
> > >  3 files changed, 67 insertions(+)
> > >
> > > diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> > > index 4ffd991..4f834c1 100644
> > > --- a/target/arm/cpu.h
> > > +++ b/target/arm/cpu.h
> > > @@ -560,6 +560,8 @@ typedef struct CPUARMState {
> > >          uint64_t esr;
> > >      } serror;
> > >
> > > +    uint8_t ext_dabt_pending; /* Request for injecting ext DABT */
> > 
> > I was trying to work out whether we need to migrate this state,
> > and I'm not sure. Andrew, do you know? I think this comes down
> > to "at what points in QEMU's kvm run loop can migration kick in",
> > and specifically if we get a KVM_EXIT_ARM_NISV do we definitely
> > go round the loop and KVM_RUN again without ever checking
> > to see if we should do a migration ?
> >
> 
> I'd prefer a migration expert confirm this, so I've CC'ed David and Juan,
> but afaict there's no way to break out of the KVM_RUN loop after a
> successful (ret=0) call to kvm_arch_handle_exit() until after the next
> KVM_RUN ioctl. This is because even if migration kicks the vcpus between
> kvm_arch_handle_exit() and the next run, the signal won't do anything
> other than prepare the vcpu for an immediate exit.

This is a level I've not looked at I'm afraid.
However, the point at which we're saving the vCPU state is when the
vCPUs are stopped; so as long as your state that you save is everything
you need to restart and you migrate that then you should be OK; but in
the end fromt he migration point of view we just stop the VM and ask for
each devices state.

Dave

> Thanks,
> drew 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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