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Message-ID: <dcf40392-26a7-b4f1-ad2c-44fac99fb330@redhat.com>
Date: Wed, 7 Sep 2022 12:21:06 +0800
From: Jason Wang <jasowang@...hat.com>
To: Guo Zhi <qtxuning1999@...u.edu.cn>, eperezma@...hat.com,
sgarzare@...hat.com, mst@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [RFC v3 1/7] vhost: expose used buffers
在 2022/9/1 13:54, Guo Zhi 写道:
> Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch
> of descriptors.
>
> Signed-off-by: Guo Zhi <qtxuning1999@...u.edu.cn>
> ---
> drivers/vhost/vhost.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 40097826cff0..26862c8bf751 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];
> + }
Would it better to have a dedicated helper like
vhost_add_used_in_order() here?
> 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;
> }
> @@ -2388,7 +2398,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> smp_wmb();
> /* Log used ring entry write. */
> log_used(vq, ((void __user *)used - (void __user *)vq->used),
> - count * sizeof *used);
> + copy_n * sizeof(*used));
> }
> old = vq->last_used_idx;
> new = (vq->last_used_idx += count);
> @@ -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)) {
This seems strange, any reason for this? (Actually if we support
in-order we only need one used slot which fit for the case here)
Thanks
> r = __vhost_add_used_n(vq, heads, n);
> if (r < 0)
> return r;
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