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Message-ID: <CACGkMEuqPtrCotXRcP2kzdaJ50L3oY7U-LVAKNuXOFJP_h1_PQ@mail.gmail.com>
Date: Mon, 17 Nov 2025 12:26:51 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net] vhost: rewind next_avail_head while discarding descriptors
On Fri, Nov 14, 2025 at 2:25 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Fri, Nov 14, 2025 at 09:53:12AM +0800, Jason Wang wrote:
> > On Thu, Nov 13, 2025 at 4:13 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Thu, Nov 13, 2025 at 09:54:20AM +0800, Jason Wang wrote:
> > > > When discarding descriptors with IN_ORDER, we should rewind
> > > > next_avail_head otherwise it would run out of sync with
> > > > last_avail_idx. This would cause driver to report
> > > > "id X is not a head".
> > > >
> > > > Fixing this by returning the number of descriptors that is used for
> > > > each buffer via vhost_get_vq_desc_n() so caller can use the value
> > > > while discarding descriptors.
> > > >
> > > > Fixes: 67a873df0c41 ("vhost: basic in order support")
> > > > Cc: stable@...r.kernel.org
> > > > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > >
> > > Wow that change really caused a lot of fallout.
> > >
> > > Thanks for the patch! Yet something to improve:
> > >
> > >
> > > > ---
> > > > drivers/vhost/net.c | 53 ++++++++++++++++++++++++++-----------------
> > > > drivers/vhost/vhost.c | 43 ++++++++++++++++++++++++-----------
> > > > drivers/vhost/vhost.h | 9 +++++++-
> > > > 3 files changed, 70 insertions(+), 35 deletions(-)
> > > >
> > > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > > > index 35ded4330431..8f7f50acb6d6 100644
> > > > --- a/drivers/vhost/net.c
> > > > +++ b/drivers/vhost/net.c
> > > > @@ -592,14 +592,15 @@ static void vhost_net_busy_poll(struct vhost_net *net,
> > > > static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
> > > > struct vhost_net_virtqueue *tnvq,
> > > > unsigned int *out_num, unsigned int *in_num,
> > > > - struct msghdr *msghdr, bool *busyloop_intr)
> > > > + struct msghdr *msghdr, bool *busyloop_intr,
> > > > + unsigned int *ndesc)
> > > > {
> > > > struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
> > > > struct vhost_virtqueue *rvq = &rnvq->vq;
> > > > struct vhost_virtqueue *tvq = &tnvq->vq;
> > > >
> > > > - int r = vhost_get_vq_desc(tvq, tvq->iov, ARRAY_SIZE(tvq->iov),
> > > > - out_num, in_num, NULL, NULL);
> > > > + int r = vhost_get_vq_desc_n(tvq, tvq->iov, ARRAY_SIZE(tvq->iov),
> > > > + out_num, in_num, NULL, NULL, ndesc);
> > > >
> > > > if (r == tvq->num && tvq->busyloop_timeout) {
> > > > /* Flush batched packets first */
> > > > @@ -610,8 +611,8 @@ static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
> > > >
> > > > vhost_net_busy_poll(net, rvq, tvq, busyloop_intr, false);
> > > >
> > > > - r = vhost_get_vq_desc(tvq, tvq->iov, ARRAY_SIZE(tvq->iov),
> > > > - out_num, in_num, NULL, NULL);
> > > > + r = vhost_get_vq_desc_n(tvq, tvq->iov, ARRAY_SIZE(tvq->iov),
> > > > + out_num, in_num, NULL, NULL, ndesc);
> > > > }
> > > >
> > > > return r;
> > > > @@ -642,12 +643,14 @@ static int get_tx_bufs(struct vhost_net *net,
> > > > struct vhost_net_virtqueue *nvq,
> > > > struct msghdr *msg,
> > > > unsigned int *out, unsigned int *in,
> > > > - size_t *len, bool *busyloop_intr)
> > > > + size_t *len, bool *busyloop_intr,
> > > > + unsigned int *ndesc)
> > > > {
> > > > struct vhost_virtqueue *vq = &nvq->vq;
> > > > int ret;
> > > >
> > > > - ret = vhost_net_tx_get_vq_desc(net, nvq, out, in, msg, busyloop_intr);
> > > > + ret = vhost_net_tx_get_vq_desc(net, nvq, out, in, msg,
> > > > + busyloop_intr, ndesc);
> > > >
> > > > if (ret < 0 || ret == vq->num)
> > > > return ret;
> > > > @@ -766,6 +769,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> > > > int sent_pkts = 0;
> > > > bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
> > > > bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
> > > > + unsigned int ndesc = 0;
> > > >
> > > > do {
> > > > bool busyloop_intr = false;
> > > > @@ -774,7 +778,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> > > > vhost_tx_batch(net, nvq, sock, &msg);
> > > >
> > > > head = get_tx_bufs(net, nvq, &msg, &out, &in, &len,
> > > > - &busyloop_intr);
> > > > + &busyloop_intr, &ndesc);
> > > > /* On error, stop handling until the next kick. */
> > > > if (unlikely(head < 0))
> > > > break;
> > > > @@ -806,7 +810,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> > > > goto done;
> > > > } else if (unlikely(err != -ENOSPC)) {
> > > > vhost_tx_batch(net, nvq, sock, &msg);
> > > > - vhost_discard_vq_desc(vq, 1);
> > > > + vhost_discard_vq_desc(vq, 1, ndesc);
> > > > vhost_net_enable_vq(net, vq);
> > > > break;
> > > > }
> > > > @@ -829,7 +833,7 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
> > > > err = sock->ops->sendmsg(sock, &msg, len);
> > > > if (unlikely(err < 0)) {
> > > > if (err == -EAGAIN || err == -ENOMEM || err == -ENOBUFS) {
> > > > - vhost_discard_vq_desc(vq, 1);
> > > > + vhost_discard_vq_desc(vq, 1, ndesc);
> > > > vhost_net_enable_vq(net, vq);
> > > > break;
> > > > }
> > > > @@ -868,6 +872,7 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> > > > int err;
> > > > struct vhost_net_ubuf_ref *ubufs;
> > > > struct ubuf_info_msgzc *ubuf;
> > > > + unsigned int ndesc = 0;
> > > > bool zcopy_used;
> > > > int sent_pkts = 0;
> > > >
> > > > @@ -879,7 +884,7 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> > > >
> > > > busyloop_intr = false;
> > > > head = get_tx_bufs(net, nvq, &msg, &out, &in, &len,
> > > > - &busyloop_intr);
> > > > + &busyloop_intr, &ndesc);
> > > > /* On error, stop handling until the next kick. */
> > > > if (unlikely(head < 0))
> > > > break;
> > > > @@ -941,7 +946,7 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> > > > vq->heads[ubuf->desc].len = VHOST_DMA_DONE_LEN;
> > > > }
> > > > if (retry) {
> > > > - vhost_discard_vq_desc(vq, 1);
> > > > + vhost_discard_vq_desc(vq, 1, ndesc);
> > > > vhost_net_enable_vq(net, vq);
> > > > break;
> > > > }
> > > > @@ -1045,11 +1050,12 @@ static int get_rx_bufs(struct vhost_net_virtqueue *nvq,
> > > > unsigned *iovcount,
> > > > struct vhost_log *log,
> > > > unsigned *log_num,
> > > > - unsigned int quota)
> > > > + unsigned int quota,
> > > > + unsigned int *ndesc)
> > > > {
> > > > struct vhost_virtqueue *vq = &nvq->vq;
> > > > bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
> > > > - unsigned int out, in;
> > > > + unsigned int out, in, desc_num, n = 0;
> > > > int seg = 0;
> > > > int headcount = 0;
> > > > unsigned d;
> > > > @@ -1064,9 +1070,9 @@ static int get_rx_bufs(struct vhost_net_virtqueue *nvq,
> > > > r = -ENOBUFS;
> > > > goto err;
> > > > }
> > > > - r = vhost_get_vq_desc(vq, vq->iov + seg,
> > > > - ARRAY_SIZE(vq->iov) - seg, &out,
> > > > - &in, log, log_num);
> > > > + r = vhost_get_vq_desc_n(vq, vq->iov + seg,
> > > > + ARRAY_SIZE(vq->iov) - seg, &out,
> > > > + &in, log, log_num, &desc_num);
> > > > if (unlikely(r < 0))
> > > > goto err;
> > > >
> > > > @@ -1093,6 +1099,7 @@ static int get_rx_bufs(struct vhost_net_virtqueue *nvq,
> > > > ++headcount;
> > > > datalen -= len;
> > > > seg += in;
> > > > + n += desc_num;
> > > > }
> > > >
> > > > *iovcount = seg;
> > > > @@ -1113,9 +1120,11 @@ static int get_rx_bufs(struct vhost_net_virtqueue *nvq,
> > > > nheads[0] = headcount;
> > > > }
> > > >
> > > > + *ndesc = n;
> > > > +
> > > > return headcount;
> > > > err:
> > > > - vhost_discard_vq_desc(vq, headcount);
> > > > + vhost_discard_vq_desc(vq, headcount, n);
> > >
> > > So here ndesc and n are the same, but below in vhost_discard_vq_desc
> > > they are different. Fun.
> >
> > Not necessarily the same, a buffer could contain more than 1 descriptor.
>
>
> *ndesc = n kinda guarantees it's the same, no?
I misread your comment, in the error path the ndesc is left unused.
Would this be a problem?
>
> > >
> > > > return r;
> > > > }
> > > >
> > > > @@ -1151,6 +1160,7 @@ static void handle_rx(struct vhost_net *net)
> > > > struct iov_iter fixup;
> > > > __virtio16 num_buffers;
> > > > int recv_pkts = 0;
> > > > + unsigned int ndesc;
> > > >
> > > > mutex_lock_nested(&vq->mutex, VHOST_NET_VQ_RX);
> > > > sock = vhost_vq_get_backend(vq);
> > > > @@ -1182,7 +1192,8 @@ static void handle_rx(struct vhost_net *net)
> > > > headcount = get_rx_bufs(nvq, vq->heads + count,
> > > > vq->nheads + count,
> > > > vhost_len, &in, vq_log, &log,
> > > > - likely(mergeable) ? UIO_MAXIOV : 1);
> > > > + likely(mergeable) ? UIO_MAXIOV : 1,
> > > > + &ndesc);
> > > > /* On error, stop handling until the next kick. */
> > > > if (unlikely(headcount < 0))
> > > > goto out;
> > > > @@ -1228,7 +1239,7 @@ static void handle_rx(struct vhost_net *net)
> > > > if (unlikely(err != sock_len)) {
> > > > pr_debug("Discarded rx packet: "
> > > > " len %d, expected %zd\n", err, sock_len);
> > > > - vhost_discard_vq_desc(vq, headcount);
> > > > + vhost_discard_vq_desc(vq, headcount, ndesc);
> > > > continue;
> > > > }
> > > > /* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
> > > > @@ -1252,7 +1263,7 @@ static void handle_rx(struct vhost_net *net)
> > > > copy_to_iter(&num_buffers, sizeof num_buffers,
> > > > &fixup) != sizeof num_buffers) {
> > > > vq_err(vq, "Failed num_buffers write");
> > > > - vhost_discard_vq_desc(vq, headcount);
> > > > + vhost_discard_vq_desc(vq, headcount, ndesc);
> > > > goto out;
> > > > }
> > > > nvq->done_idx += headcount;
> > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > index 8570fdf2e14a..b56568807588 100644
> > > > --- a/drivers/vhost/vhost.c
> > > > +++ b/drivers/vhost/vhost.c
> > > > @@ -2792,18 +2792,11 @@ static int get_indirect(struct vhost_virtqueue *vq,
> > > > return 0;
> > > > }
> > > >
> > > > -/* This looks in the virtqueue and for the first available buffer, and converts
> > > > - * it to an iovec for convenient access. Since descriptors consist of some
> > > > - * number of output then some number of input descriptors, it's actually two
> > > > - * iovecs, but we pack them into one and note how many of each there were.
> > > > - *
> > > > - * This function returns the descriptor number found, or vq->num (which is
> > > > - * never a valid descriptor number) if none was found. A negative code is
> > > > - * returned on error. */
> > >
> > > A new module API with no docs at all is not good.
> > > Please add documentation to this one. vhost_get_vq_desc
> > > is a subset and could refer to it.
> >
> > Fixed.
> >
> > >
> > > > -int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> > > > - struct iovec iov[], unsigned int iov_size,
> > > > - unsigned int *out_num, unsigned int *in_num,
> > > > - struct vhost_log *log, unsigned int *log_num)
> > > > +int vhost_get_vq_desc_n(struct vhost_virtqueue *vq,
> > > > + struct iovec iov[], unsigned int iov_size,
> > > > + unsigned int *out_num, unsigned int *in_num,
> > > > + struct vhost_log *log, unsigned int *log_num,
> > > > + unsigned int *ndesc)
> > >
> > > > {
> > > > bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
> > > > struct vring_desc desc;
> > > > @@ -2921,16 +2914,40 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> > > > vq->last_avail_idx++;
> > > > vq->next_avail_head += c;
> > > >
> > > > + if (ndesc)
> > > > + *ndesc = c;
> > > > +
> > > > /* Assume notifications from guest are disabled at this point,
> > > > * if they aren't we would need to update avail_event index. */
> > > > BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
> > > > return head;
> > > > }
> > > > +EXPORT_SYMBOL_GPL(vhost_get_vq_desc_n);
> > > > +
> > > > +/* This looks in the virtqueue and for the first available buffer, and converts
> > > > + * it to an iovec for convenient access. Since descriptors consist of some
> > > > + * number of output then some number of input descriptors, it's actually two
> > > > + * iovecs, but we pack them into one and note how many of each there were.
> > > > + *
> > > > + * This function returns the descriptor number found, or vq->num (which is
> > > > + * never a valid descriptor number) if none was found. A negative code is
> > > > + * returned on error.
> > > > + */
> > > > +int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> > > > + struct iovec iov[], unsigned int iov_size,
> > > > + unsigned int *out_num, unsigned int *in_num,
> > > > + struct vhost_log *log, unsigned int *log_num)
> > > > +{
> > > > + return vhost_get_vq_desc_n(vq, iov, iov_size, out_num, in_num,
> > > > + log, log_num, NULL);
> > > > +}
> > > > EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
> > > >
> > > > /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
> > > > -void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
> > > > +void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n,
> > > > + unsigned int ndesc)
> > >
> > > ndesc is number of descriptors? And n is what, in that case?
> >
> > The semantic of n is not changed which is the number of buffers, ndesc
> > is the number of descriptors.
>
> History is not that relevant. To make the core readable pls
> change the names to readable ones.
>
> Specifically n is really nbufs, maybe?
Right.
Thanks
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