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

[Jan Kiszka <jan.kiszka@web.de>]

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Am 07.01.2011 18:53, Gleb Natapov wrote:
> On Fri, Jan 07, 2011 at 06:30:57PM +0100, Jan Kiszka wrote:
>> 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).
>>
> Yes, I understand the build requirement. Runtime behaviour should not
> change.

Yep, for sure.

BTW, the self-IPI on pending exit request is there for a reason I but.
In order to complete half-done string-io or something like that? Would
be the next patch for upstream then.

> 
>> 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.
>>
> I thought there was an agreement to accept qemu-kvm implementation as is
> into upstream (without some parts like device assignment). If you look
> at qemu-kvm you'll see that upstream implementation is marked as
> OBSOLETE_KVM_IMPL.

You can't merge both trees without introducing regressions, either in
the kvm part or some other section that qemu-kvm did not stress. IMO,
there is no way around understanding all the nice little "fixes" that
piled up over the years and translate them into proper, documented patches.

> 
>> 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.
>>
> You are optimistic. My prediction is that it will take at least one major RHEL
> release until such merged code base will become production-grade. That
> is when most bugs that were introduced by eliminating subtle differences
> between working and non-working version will be found :)

The more upstream code qemu-kvm stresses, the faster this convergence
will become. And there is really not that much left. E.g, I've a
qemu-kvm-x86.c here that is <400 LOC.

> 
> BTW Do you have a plan how to move upstream to thread per vcpu?

Upstream has this already, but it's - once again - a different
implementation. Understanding those differences is one of the next steps.

In fact, as posted recently, unifying the execution model
implementations is the only big problem I see. In-kernel irqchips and
device assignment are things that can live in qemu-kvm without much
conflicts until they are finally mergable.

Jan


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