Re: kern/58049: vioif(4) panics if the virtq size is too small

[Jason Thorpe <thorpej%me.com@localhost>]

The following reply was made to PR kern/58049; it has been noted by GNATS.

From: Jason Thorpe <thorpej%me.com@localhost>
To: Tetsuya Isaki <isaki%pastel-flower.jp@localhost>
Cc: kern-bug-people%netbsd.org@localhost,
 gnats-admin%netbsd.org@localhost,
 netbsd-bugs%netbsd.org@localhost,
 gnats-bugs%netbsd.org@localhost
Subject: Re: kern/58049: vioif(4) panics if the virtq size is too small
Date: Mon, 18 Mar 2024 08:51:45 -0700

 > On Mar 18, 2024, at 6:00 AM, isaki%pastel-flower.jp@localhost wrote:
 >=20
 > How about the attached patch?  It works to me.
 
 This looks mostly fine but I have one question...
 
 > --- src/sys/dev/pci/if_vioif.c
 > +++ src/sys/dev/pci/if_vioif.c
 > @@ -1993,6 +1993,8 @@ vioif_send_common_locked(struct ifnet *ifp, =
 struct vioif_netqueue *netq,
 >=20
 > for (;;) {
 > int slot, r;
 > + bool need_defrag;
 > +
 > r =3D virtio_enqueue_prep(vsc, vq, &slot);
 > if (r =3D=3D EAGAIN) {
 > txc->txc_no_free_slots =3D true;
 > @@ -2014,8 +2016,17 @@ vioif_send_common_locked(struct ifnet *ifp, =
 struct vioif_netqueue *netq,
 > map =3D &netq->netq_maps[slot];
 > KASSERT(map->vnm_mbuf =3D=3D NULL);
 >=20
 > + need_defrag =3D false;
 > r =3D vioif_net_load_mbuf(vsc, map, m, BUS_DMA_WRITE);
 > if (r !=3D 0) {
 > + need_defrag =3D true;
 > + } else if (map->vnm_mbuf_map->dm_nsegs >=3D vq->vq_maxnsegs) {
 > + /* loaded but too many segments for VirtQ */
 
 When the DMA map is created, do we not yet know the max number of =
 segments?  Or is the problem that different queues might have different =
 segment counts and the DMA maps are shared between them?  In a perfect =
 world, this extra test isn't needed because the bus_dma back-end honors =
 the device's maximum segment count.
 
 -- thorpej