On Thu, Mar 12, 2026 at 09:08:33AM +0100, Laurent Vivier wrote:
> On 3/12/26 05:39, David Gibson wrote:
> > On Mon, Mar 09, 2026 at 10:47:44AM +0100, Laurent Vivier wrote:
> > > iPXE places the vnet header in one virtqueue descriptor and the payload
> > > in another. When passt maps these descriptors, it needs two iovecs per
> > > virtqueue element to handle this layout.
> > > 
> > > Without this, passt crashes with:
> > > 
> > >    ASSERTION FAILED in virtqueue_map_desc (virtio.c:403): num_sg < max_num_sg
> > > 
> > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > > ---
> > >   udp_vu.c    |  8 ++++----
> > >   vu_common.c | 15 +++++++++++----
> > >   2 files changed, 15 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/udp_vu.c b/udp_vu.c
> > > index cb5274aa1d81..47659b0402fd 100644
> > > --- a/udp_vu.c
> > > +++ b/udp_vu.c
> > > @@ -34,7 +34,7 @@
> > >   #include "vu_common.h"
> > >   static struct iovec     iov_vu		[VIRTQUEUE_MAX_SIZE];
> > > -static struct vu_virtq_element	elem		[VIRTQUEUE_MAX_SIZE];
> > > +static struct vu_virtq_element	elem	[VIRTQUEUE_MAX_SIZE / IOV_PER_ELEM];
> > 
> > The two level structure of the queues (array of elems, each pointing
> > to an array of iovs) confuses me a bit.  I would have thought that
> > VIRTQUEUE_MAX_SIZE represented the maximum number of elements, even if
> > those elements had multiple iovs.  Can you enlighten me?
> > 
> In all the code until now I supposed the number of iovec is equal to the
> number of element because we had only one iovec per element.
> 
> At virtio level, VIRTQUEUE_MAX_SIZE is the number of vring_desc in a
> virtqueue, and a vring_desc is translated to an iovec in
> virtqueue_map_desc() that are collected to an element in
> vu_queue_map_desc(). So in passt, the maximum number of collected iovec will
> be
> VIRTQUEUE_MAX_SIZE and the maximum of element is VIRTQUEUE_MAX_SIZE if we
> have only one iovec per element.

Ok... sorry, I'm still unclear.  Is the "element" a concept that's
part of the "hardware" interface?  Or just part of the structure on
the software side?

> I'm reworking all of this to be able to have several iovec per element on
> demand (in vu_queue_map_desc()). This patch and previous one will be
> dropped.

Ok.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson