----- Original Message ----- > From: "eperezma" <eperezma@xxxxxxxxxx> > To: "Guo Zhi" <qtxuning1999@xxxxxxxxxxx> > Cc: "jasowang" <jasowang@xxxxxxxxxx>, "sgarzare" <sgarzare@xxxxxxxxxx>, "Michael Tsirkin" <mst@xxxxxxxxxx>, "netdev" > <netdev@xxxxxxxxxxxxxxx>, "linux-kernel" <linux-kernel@xxxxxxxxxxxxxxx>, "kvm list" <kvm@xxxxxxxxxxxxxxx>, > "virtualization" <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx> > Sent: Thursday, August 18, 2022 2:16:40 PM > Subject: Re: [RFC v2 1/7] vhost: expose used buffers > On Wed, Aug 17, 2022 at 3:58 PM Guo Zhi <qtxuning1999@xxxxxxxxxxx> wrote: >> >> Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch >> of descriptors. >> >> Signed-off-by: Guo Zhi <qtxuning1999@xxxxxxxxxxx> >> --- >> drivers/vhost/vhost.c | 14 ++++++++++++-- >> drivers/vhost/vhost.h | 1 + >> 2 files changed, 13 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index 40097826cff0..7b20fa5a46c3 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -2376,10 +2376,20 @@ static int __vhost_add_used_n(struct vhost_virtqueue >> *vq, >> vring_used_elem_t __user *used; >> u16 old, new; >> int start; >> + int copy_n = count; >> >> + /** >> + * If in order feature negotiated, devices can notify the use of a batch >> of buffers to >> + * the driver by only writing out a single used ring entry with the id >> corresponding >> + * to the head entry of the descriptor chain describing the last buffer >> in the batch. >> + */ >> + if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) { >> + copy_n = 1; >> + heads = &heads[count - 1]; >> + } >> start = vq->last_used_idx & (vq->num - 1); >> used = vq->used->ring + start; >> - if (vhost_put_used(vq, heads, start, count)) { >> + if (vhost_put_used(vq, heads, start, copy_n)) { >> vq_err(vq, "Failed to write used"); >> return -EFAULT; >> } >> @@ -2410,7 +2420,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct >> vring_used_elem *heads, >> >> start = vq->last_used_idx & (vq->num - 1); >> n = vq->num - start; >> - if (n < count) { >> + if (n < count && !vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) { >> r = __vhost_add_used_n(vq, heads, n); >> if (r < 0) >> return r; > > It would be simpler to use vhost_add_used in vsock/vhost_test to add a > batch of used descriptors, and leave vhost_add_used_n untouched. > > Since it's the upper layer the one that manages the in_order in this > version, we could: > * Always call vhost_add_used(vq, last_head_of_batch, ...) for the tx > queue, that does not need used length info. > * Call vhost_add_used_n for the rx queue, since the driver needs the > used length info. Very insightful view! At first, vhost_add_used will write used ring for skipped buffer in a batch, so we can't let vhost unmodified to enable in order feature. Secondly, Current changes to vhost_add_used_n will affect the rx queue, as driver have to get each buffer's length from used ring. So I would propose a flag parameter in vhost_add_used_n to decide whether the batch for in order buffer is done or nor. >> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h >> index d9109107af08..0d5c49a30421 100644 >> --- a/drivers/vhost/vhost.h >> +++ b/drivers/vhost/vhost.h >> @@ -236,6 +236,7 @@ enum { >> VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) | >> (1ULL << VIRTIO_RING_F_INDIRECT_DESC) | >> (1ULL << VIRTIO_RING_F_EVENT_IDX) | >> + (1ULL << VIRTIO_F_IN_ORDER) | >> (1ULL << VHOST_F_LOG_ALL) | >> (1ULL << VIRTIO_F_ANY_LAYOUT) | >> (1ULL << VIRTIO_F_VERSION_1) >> -- >> 2.17.1 >>