Re: [Qemu-devel] [Qemu-stable] [PULL 10/25] virtio_error: don't invoke status callbacks

[Peter Lieven]

Am 13.02.2018 um 23:23 schrieb Michael S. Tsirkin:
> On Tue, Feb 13, 2018 at 09:53:58PM +0100, Peter Lieven wrote:
>> Am 21.12.2017 um 15:29 schrieb Michael S. Tsirkin:
>>> Backends don't need to know what frontend requested a reset,
>>> and notifying then from virtio_error is messy because
>>> virtio_error itself might be invoked from backend.
>>>
>>> Let's just set the status directly.
>>>
>>> Cc: address@hidden
>>> Reported-by: Ilya Maximets <address@hidden>
>>> Signed-off-by: Michael S. Tsirkin <address@hidden>
>>> ---
>>>  hw/virtio/virtio.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
>>> index ad564b0..d6002ee 100644
>>> --- a/hw/virtio/virtio.c
>>> +++ b/hw/virtio/virtio.c
>>> @@ -2469,7 +2469,7 @@ void GCC_FMT_ATTR(2, 3) virtio_error(VirtIODevice 
>>> *vdev, const char *fmt, ...)
>>>      va_end(ap);
>>>  
>>>      if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
>>> -        virtio_set_status(vdev, vdev->status | 
>>> VIRTIO_CONFIG_S_NEEDS_RESET);
>>> +        vdev->status = vdev->status | VIRTIO_CONFIG_S_NEEDS_RESET;
>>>          virtio_notify_config(vdev);
>>>      }
>>>  
>>
>> Is it possible that this patch introduces a stall in I/O and a deadlock on a 
>> drain all?
>>
>> I have seen Qemu VMs being I/O stalled and deadlocking on a vm stop command 
>> in
>>
>> blk_drain_all. This happened after a longer storage outage.
>>
>>
>> I am asking just theoretically because I have seen this behaviour first when 
>> we
>>
>> backported this patch in our stable 2.9 branch.
>>
>>
>> Thank you,
>>
>> Peter
> Well - this patch was introduced to fix a crash, but
> a well behaved VM should not trigger VIRTIO_CONFIG_S_NEEDS_RESET -
> did you see any error messages in the log when this triggered?

You mean in the guest or on the host? On the host I have seen nothing.

I actually did not know the reasoning behing this patch. I was just searching 
for an explaination
for the strange I/O stalls that I have seen.

And it was not only one guest but a few hundreds. So I think I have to search 
for another cause.

Thank you,
Peter