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Message-ID: <CACGkMEtnihOt=g+zs0gVQ=wnx8_YF_F=QSuLQ4RGWBVuOeFi7w@mail.gmail.com>
Date: Fri, 14 Nov 2025 09:53:12 +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 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.
>
> > 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.
>
>
> > {
> > + vq->next_avail_head -= ndesc;
> > vq->last_avail_idx -= n;
> > }
> > EXPORT_SYMBOL_GPL(vhost_discard_vq_desc);
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 621a6d9a8791..69a39540df3d 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -230,7 +230,14 @@ int vhost_get_vq_desc(struct vhost_virtqueue *,
> > struct iovec iov[], unsigned int iov_size,
> > unsigned int *out_num, unsigned int *in_num,
> > struct vhost_log *log, unsigned int *log_num);
> > -void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
> > +
> > +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);
> > +
> > +void vhost_discard_vq_desc(struct vhost_virtqueue *, int n, unsigned int ndesc);
> >
> > bool vhost_vq_work_queue(struct vhost_virtqueue *vq, struct vhost_work *work);
> > bool vhost_vq_has_work(struct vhost_virtqueue *vq);
> > --
> > 2.31.1
>
Thanks
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