Re: [Qemu-devel] [PATCH] tcg: handle EXCP_ATOMIC exception properly

["Alex Bennée" <alex.bennee@linaro.org>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 10/02/2017 13:13, Alex Bennée wrote:
>>> +        if (atomic_read(&other_cpu->running) &&
>>> !qemu_cpu_is_self(other_cpu)) {
>> The comment above reads:
>>
>>   Must only be called from outside cpu_exec.
>>
>> So we need to revise this comment. Is this really a limitation or was it
>> originally the design goal?
>
> If you want to call it within cpu_exec, you can always use
> cpu_exec_end/start around it.
>
> I think we should first of all get rid of the iothread lock within
> cpu_exec, and then look at this patch again.

What to do with the MTTCG series in the meantime?

I'm hesitant to hold up the whole series for a potentially messy
re-factor of the cpu loop code to push out the BQL which could take some
time, although of course I don't want to merge something that makes that
harder.

That said for TCG system emulation EXCP_ATOMIC is currently broken with
respect to debugging. However for the initial guests/host combination of
ARMv7/8 on x86_64 we don't need the fallback with pretty much 99% of
deployed hosts. How about the following:

  - drop Pranith's patch for the current MTTCG series
  - replace with an error/abort on EXCP_ATOMIC
  - proceed with merge as plan
  - tackle the BQL lock next (along with more MTTCG guest/targets enablement)

Richard/Peter,

Any thoughts/opinions on this?

--
Alex Bennée