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

[Andrew Jones]

On Fri, Apr 17, 2020 at 11:39:25AM +0100, Peter Maydell wrote:
> On Mon, 23 Mar 2020 at 11:32, Beata Michalska
> <address@hidden> 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 <address@hidden>
> > ---
> >  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.

Thanks,
drew