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

[Stefan Berger]

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