From: David Gibson <david@gibson.dropbear.id.au>
To: Laurent Vivier <lvivier@redhat.com>
Cc: passt-dev@passt.top
Subject: Re: [PATCH v2 13/13] vhost-user,udp: Use 2 iovec entries per element
Date: Thu, 12 Mar 2026 20:47:51 +1100 [thread overview]
Message-ID: <abKLxyMZ2tjzn_PW@zatzit> (raw)
In-Reply-To: <43363999-76cf-4f4c-a73d-450c5906e6b9@redhat.com>
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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
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next prev parent reply other threads:[~2026-03-12 10:19 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-09 9:47 [PATCH v2 00/13] vhost-user,udp: Handle multiple iovec entries per virtqueue element Laurent Vivier
2026-03-09 9:47 ` [PATCH v2 01/13] iov: Add iov_truncate() helper and use it in vu handlers Laurent Vivier
2026-03-09 9:47 ` [PATCH v2 02/13] vhost-user: Centralise 802.3 frame padding in vu_collect() and vu_flush() Laurent Vivier
2026-03-12 2:05 ` David Gibson
2026-03-09 9:47 ` [PATCH v2 03/13] vhost-user: Use ARRAY_SIZE(elem) instead of VIRTQUEUE_MAX_SIZE Laurent Vivier
2026-03-09 9:47 ` [PATCH v2 04/13] udp_vu: Use iov_tail to manage virtqueue buffers Laurent Vivier
2026-03-12 2:38 ` David Gibson
2026-03-09 9:47 ` [PATCH v2 05/13] udp_vu: Move virtqueue management from udp_vu_sock_recv() to its caller Laurent Vivier
2026-03-12 3:44 ` David Gibson
2026-03-09 9:47 ` [PATCH v2 06/13] iov: Add IOV_PUT_HEADER() to write header data back to iov_tail Laurent Vivier
2026-03-12 4:12 ` David Gibson
2026-03-12 8:20 ` Laurent Vivier
2026-03-09 9:47 ` [PATCH v2 07/13] udp: Pass iov_tail to udp_update_hdr4()/udp_update_hdr6() Laurent Vivier
2026-03-12 4:16 ` David Gibson
2026-03-09 9:47 ` [PATCH v2 08/13] udp_vu: Use iov_tail in udp_vu_prepare() Laurent Vivier
2026-03-12 4:30 ` David Gibson
2026-03-12 8:19 ` Laurent Vivier
2026-03-12 9:51 ` David Gibson
2026-03-09 9:47 ` [PATCH v2 09/13] vu_common: Pass iov_tail to vu_set_vnethdr() Laurent Vivier
2026-03-12 4:34 ` David Gibson
2026-03-09 9:47 ` [PATCH v2 10/13] vu_common: Accept explicit iovec counts in vu_set_element() Laurent Vivier
2026-03-09 9:47 ` [PATCH v2 11/13] vu_common: Accept explicit iovec count per element in vu_init_elem() Laurent Vivier
2026-03-09 9:47 ` [PATCH v2 12/13] vu_common: Prepare to use multibuffer with guest RX Laurent Vivier
2026-03-09 9:47 ` [PATCH v2 13/13] vhost-user,udp: Use 2 iovec entries per element Laurent Vivier
2026-03-12 4:39 ` David Gibson
2026-03-12 8:08 ` Laurent Vivier
2026-03-12 9:47 ` David Gibson [this message]
2026-03-12 10:42 ` Laurent Vivier
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