[Qemu-devel] Re: qemu-kvm vs. qemu: Terminate cpu loop on reset?

[Jan Kiszka]

Am 07.01.2011 18:16, Gleb Natapov wrote:
> On Fri, Jan 07, 2011 at 05:59:34PM +0100, Jan Kiszka wrote:
>> Am 07.01.2011 17:53, Gleb Natapov wrote:
>>> On Fri, Jan 07, 2011 at 04:57:31PM +0100, Jan Kiszka wrote:
>>>> Hi,
>>>>
>>>> does anyone immediately know if this hunk from vl.c
>>>>
>>>> @@ -1278,6 +1197,10 @@ void qemu_system_reset_request(void)
>>>>      } else {
>>>>          reset_requested = 1;
>>>>      }
>>>> +    if (cpu_single_env) {
>>>> +        cpu_single_env->stopped = 1;
>>>> +        cpu_exit(cpu_single_env);
>>>> +    }
>>>>      qemu_notify_event();
>>>>  }
>>>>
>>>> is (semantically) relevant for upstream as well? IIUC, it ensures that
>>>> the kvm cpu loop is not continued if an IO access called into
>>>> qemu_system_reset_request.
>>>>
>>> I don't know TCG enough to tell. If TCG can continue vcpu execution
>>> after io without checking reset_requested then it is relevant for
>>> upstream too.
>>
>> I was first of all thinking about kvm upstream, but their handling
>> differ much less upstream than in current qemu-kvm. Anyway, need to dig
>> into the details.
>>
>>>
>>>> If yes, then it would be a good time to push a patch: these bits will
>>>> fall to dust on next merge from upstream (vl.c no longer has access to
>>>> the cpu state).
>>>>
>>> On a next merge cpu state will have to be exposed to vl.c then. This
>>> code cannot be dropped in qemu-kvm.
>>
>> I think a cleaner approach, even if it's only temporarily required, is
>> to move that code to cpus.c. That's likely also the way when we need it
>> upstream. 
> It doesn't matter where the code resides as long as it is called on
> reset.

It technically matters for the build process (vl.c is built once these
days, cpus.c is built per target).

In any case, we apparently need to fix upstream, I'm playing with some
approach.

> 
>>            If upstream does not need it, we have to understand why and
>> maybe adopt its pattern (the ultimate goal is unification anyway).
>>
> I don't consider kvm upstream as working product. The goal should be
> moving to qemu-kvm code in upstream preserving all the knowledge we
> acquired while making it production grade code.

We had this discussion before. My goal remains to filter the remaining
upstream fixes out of the noise, adjust both versions so that they are
apparently identical, and then switch to a single version.

We are on a good track now. I predict that we will be left with only one
or two major additional features in qemu-kvm in a few months from now,
no more duplications with subtle differences, and production-grade kvm
upstream stability.

Jan

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