Re: [Qemu-devel] Re: Errors on MMIO read access on VM suspend / resume operations

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 01/24/2011 05:34 PM, Jan Kiszka wrote:
> On 2011-01-24 19:27, Stefan Berger wrote:
>> On 01/18/2011 03:53 AM, Jan Kiszka wrote:
>>> On 2011-01-18 04:03, Stefan Berger wrote:
>>>> On 01/16/2011 09:43 AM, Avi Kivity wrote:
>>>>> On 01/14/2011 09:27 PM, Stefan Berger wrote:
>>>>>>> Can you sprinkle some printfs() arount kvm_run (in qemu-kvm.c) to
>>>>>>> verify this?
>>>>>>>
>>>>>> Here's what I did:
>>>>>>
>>>>>>
>>>>>> interrupt exit requested
>>>>> It appears from this you're using qemu.git.  Please try qemu-kvm.git,
>>>>> where the code appears to be correct.
>>>>>
>>>> Cc'ing qemu-devel now. For reference, here the initial problem
>>>> description:
>>>>
>>>> http://www.spinics.net/lists/kvm/msg48274.html
>>>>
>>>> I didn't know there was another tree...
>>>>
>>>> I have seen now a couple of suspends-while-reading with patches applied
>>>> to the qemu-kvm.git tree and indeed, when run with the same host kernel
>>>> and VM I do not see the debugging dumps due to double-reads that I would
>>>> have anticipated seeing by now. Now what? Can this be easily fixed in
>>>> the other Qemu tree as well?
>>> Please give this a try:
>>>
>>> git://git.kiszka.org/qemu-kvm.git queues/kvm-upstream
>>>
>>> I bet (&   hope) "kvm: Unconditionally reenter kernel after IO exits"
>>> fixes the issue for you. If other problems pop up with that tree, also
>>> try resetting to that particular commit.
>>>
>>> I'm currently trying to shake all those hidden or forgotten bug fixes
>>> out of qemu-kvm and port them upstream. Most of those subtle differences
>>> should hopefully soon be history.
>>>
>> I did the same test as I did with Avi's tree and haven't seen the
>> consequences of possible double-reads. So, I would say that you should
>> upstream those patches...
>>
>> I searched for the text you mention above using 'gitk' but couldn't find
>> a patch with that headline in your tree. There were others that seem to
>> be related:
>>
>> Gleb Natapov: "do not enter vcpu again if it was stopped during IO"
> Err, I don't think you checked out queues/kvm-upstream. I bet you just
> ran my master branch which is a version of qemu-kvm's master. Am I right? :)
>

You're right. :-) my lack of git knowledge -  checked out the branch now.

I redid the testing and it passed. No double-reads and lost bytes from 
what I could see.

>>>> One thing I'd like to mention is that I have seen what I think are
>>>> interrupt stalls when running my tests inside the qemu-kvm.git tree
>>>> version and not suspending at all. A some point the interrupt counter in
>>>> the guest kernel does not increase anymore even though I see the device
>>>> model raising the IRQ and lowering it. The same tests run literally
>>>> forever in the qemu.git tree version of Qemu.
>>> What about qemu-kmv and -no-kvm-irqchip?
>> That seems to be necessary for both trees, yours and the one Avi pointed
>> me to. If applied, then I did not see the interrupt problem.
> And the fact that you were able to call qemu from my tree with
> -no-kvm-irqchip just underlines my assumption: that switch is refused by
> upstream. Please retry with the latest kvm-upstream queue.
>
> Besides that, this other bug you may see in the in-kernel IRQ path - how
> can we reproduce it?
Unfortunately I don't know. Some things have to come together for the 
code I am working on to become available and useful for everyone. It's 
going to be a while.

Thanks!
    Stefan
> Jan
>