Re: [PATCH] vu_common: Always set num_buffers in virtio-net header

[Stefano Brivio <sbrivio@redhat.com>]

On Tue,  3 Mar 2026 16:17:34 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> Legacy virtio used two different header formats: struct virtio_net_hdr
> (10 bytes) when VIRTIO_NET_F_MRG_RXBUF was not negotiated, and
> struct virtio_net_hdr_mrg_rxbuf (12 bytes) when it was.  The
> num_buffers field only existed in the larger header.
> 
> Modern virtio (VIRTIO_F_VERSION_1, i.e. virtio 1.0+) always uses the
> 12-byte struct virtio_net_hdr_mrg_rxbuf header regardless of whether
> VIRTIO_NET_F_MRG_RXBUF is negotiated, so num_buffers is always present
> in the header.  passt only supports modern virtio and dies if
> VIRTIO_F_VERSION_1 is not negotiated (vhost_user.c), and VNET_HLEN is
> unconditionally defined as sizeof(struct virtio_net_hdr_mrg_rxbuf).
> 
> The virtio specification (v1.1, section 5.1.6) requires that:
> 
>   "The device MUST set num_buffers to 1 if VIRTIO_NET_F_MRG_RXBUF has
>    not been negotiated."
> 
> vu_set_vnethdr() only set num_buffers when VIRTIO_NET_F_MRG_RXBUF was
> negotiated.  When it was not, num_buffers was left uninitialised,
> violating the spec.
> 
> Since vu_collect() already limits buffer collection to a single element
> when VIRTIO_NET_F_MRG_RXBUF is not negotiated, num_buffers passed by
> callers is guaranteed to be 1 in that case.  We can therefore
> unconditionally set num_buffers, which makes the vdev parameter
> unnecessary.
> 
> Drop the vdev parameter from vu_set_vnethdr() and update all callers.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Applied.

-- 
Stefano