Re: [PATCH v4 00/10] vhost-user: Preparatory series for multiple iovec entries per virtqueue element

[Laurent Vivier <lvivier@redhat.com>]

On 5/21/26 10:30, Laurent Vivier wrote:
> On 5/20/26 22:53, Stefano Brivio wrote:
>> On Wed, 20 May 2026 18:18:52 +0200
>> Stefano Brivio <sbrivio@redhat.com> wrote:
>>
>>> On Wed, 20 May 2026 18:07:08 +0200
>>> Stefano Brivio <sbrivio@redhat.com> wrote:
>>>
>>>> On Wed, 20 May 2026 17:34:45 +0200
>>>> Stefano Brivio <sbrivio@redhat.com> wrote:
>>>>> On Wed, 13 May 2026 13:52:08 +0200
>>>>> Laurent Vivier <lvivier@redhat.com> wrote:
>>>>>> Currently, the vhost-user path assumes each virtqueue element contains
>>>>>> exactly one iovec entry covering the entire frame.  This assumption
>>>>>> breaks as some virtio-net drivers (notably iPXE) provide descriptors where the
>>>>>> vnet header and the frame payload are in separate buffers, resulting in
>>>>>> two iovec entries per virtqueue element.
>>>>>>
>>>>>> This series refactors the vhost-user data path so that frame lengths,
>>>>>> header sizes, and padding are tracked and passed explicitly rather than
>>>>>> being derived from iovec sizes.  This decoupling is a prerequisite for
>>>>>> correctly handling padding of multi-buffer frames.
>>>>>
>>>>> Sorry to bring (likely) bad news, but this series seems to introduce a
>>>>> regression: I got the migration/rampstream_in tests fail twice in a
>>>>> row, which I've never saw happening (I think I saw a single failure a
>>>>> long time ago when the machine had a high CPU load, but nothing else).
>>>>>
>>>>> I'm currently bisecting and the bisect seems to point towards the end
>>>>> of the series (probably 10/10), but I haven't finished yet. I'll keep
>>>>> you posted. I haven't spotted anything that might cause issues there.
>>>>
>>>> Yeah, that's the one :(
>>>>
>>>> $ git bisect bad
>>>> db798fc60f4c5869cb53168354e068fb4dabd91a is the first bad commit
>>>> commit db798fc60f4c5869cb53168354e068fb4dabd91a
>>>> Author: Laurent Vivier <lvivier@redhat.com>
>>>> Date:   Wed May 13 13:52:18 2026 +0200
>>>>
>>>>      vhost-user: Centralise Ethernet frame padding in vu_collect() and vu_pad()
> 
> I checked on my system with the commit previous to this series,
> bcc3d37a6e01 ("util: Fix changes to assert_with_msg()") and rampstream_in fails too (not 
> everytime).
> 
>  > TCP/IPv4: sequence check, ramps, inbound
> ...failed.
> 
> and rampstream_out hangs sometime too.
> 
> I'm going to try with ealier commits.

For me the problem can happen with any commit...

As it depends on the execution path and on the load and speed of the system it looks like 
a race condition.

Did you try to test on a host with a kernel patched with
"[PATCH net v2 0/2] Fix race condition between TCP_REPAIR dump and data receive" ?

Thanks,
Laurent