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

From: Peter Maydell
Subject: Re: [PATCH v4 1/2] target/arm: kvm: Handle DABT with no valid ISS
Date: Fri, 24 Apr 2020 13:51:41 +0100

On Fri, 24 Apr 2020 at 13:17, Dr. David Alan Gilbert
<address@hidden> wrote:
> * Andrew Jones (address@hidden) wrote:
> > On Fri, Apr 17, 2020 at 11:39:25AM +0100, Peter Maydell wrote:
> > > 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.

Yeah, but I think the question is "at what point in the main loop
do we check 'is the VM stopping'". It sounds from what Andrew
says that the answer is that it can't happen at this point in
time, though.

-- PMM

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