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

[Akihiko Odaki <akihiko.odaki@daynix.com>]

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.
>>
>> thanks
>> -- PMM
> 
> 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.
> 
> Regards,
> Akihiko Odaki

Hi,

QEMU 8.0 is already released so I think it's time to revisit this.

Regards,
Akihiko Odaki