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

[Stefano Brivio <sbrivio@redhat.com>]

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()

The "TCP/IPv4: sequence check, ramps, inbound" test in rampstream_in
gets stuck, once the source is done with the migration, and passt on the
destination just printed:

Accepted TCP_REPAIR helper, PID 13
accepted connection from PID 16

I'll get captures and logs next. It seems to fail most of the times,
I had two failures in a row again.

-- 
Stefano