Re: [PATCH] accel/kvm/kvm-all: Handle register access errors

[Akihiko Odaki]

On 2023/06/19 21:19, Peter Maydell wrote:
> On Sat, 10 Jun 2023 at 04:51, Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> > On 2022/12/01 20:00, Akihiko Odaki wrote:
> > > On 2022/12/01 19:40, Peter Maydell wrote:
> > > > On Thu, 1 Dec 2022 at 10:27, Akihiko Odaki <akihiko.odaki@daynix.com>
> > > > wrote:
> > > > > A register access error typically means something seriously wrong
> > > > > happened so that anything bad can happen after that and recovery is
> > > > > impossible.
> > > > > Even failing one register access is catastorophic as
> > > > > architecture-specific code are not written so that it torelates such
> > > > > failures.
> > > > >
> > > > > Make sure the VM stop and nothing worse happens if such an error occurs.
> > > > >
> > > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > >
> > > > In a similar vein there was also
> > > > https://lore.kernel.org/all/20220617144857.34189-1-peterx@redhat.com/
> > > > back in June, which on the one hand was less comprehensive but on
> > > > the other does the plumbing to pass the error upwards rather than
> > > > reporting it immediately at point of failure.
> > > >
> > > > I'm in principle in favour but suspect we'll run into some corner
> > > > cases where we were happily ignoring not-very-important failures
> > > > (eg if you're running Linux as the host OS on a Mac M1 and your
> > > > host kernel doesn't have this fix:
> > > > https://lore.kernel.org/all/YnHz6Cw5ONR2e+KA@google.com/T/
> > > > then QEMU will go from "works by sheer luck" to "consistently
> > > > hits this error check"). So we should aim to land this extra
> > > > error checking early in the release cycle so we have plenty of
> > > > time to deal with any bug reports we get about it.
>
> > > Actually I found this problem when I tried to run QEMU with KVM on M2
> > > MacBook Air and encountered a failure described and fixed at:
> > > https://lore.kernel.org/all/20221201104914.28944-2-akihiko.odaki@daynix.com/
> > >
> > > Although the affected register was not really important, QEMU couldn't
> > > run the guest well enough because kvm_arch_put_registers for ARM64 is
> > > written in a way that it fails early. I guess the situation is not so
> > > different for other architectures as well.
> > >
> > > I still agree that this should be postponed until a new release cycle
> > > starts as register saving/restoring is too important to fail.
>
> > Hi,
> >
> > QEMU 8.0 is already released so I think it's time to revisit this.
>
> Two months ago would have been a better time :-) We're heading up
> towards softfreeze for 8.1 in about three weeks from now.
>
> thanks
> -- PMM

Hi Peter,

Please apply this.

Regards,
Akihiko Odaki