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Message-ID: <20240328052814-mutt-send-email-mst@kernel.org>
Date: Thu, 28 Mar 2024 05:31:23 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Gavin Shan <gshan@...hat.com>
Cc: virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
jasowang@...hat.com, will@...nel.org, davem@...emloft.net,
stefanha@...hat.com, sgarzare@...hat.com, keirf@...gle.com,
yihyu@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v3 3/3] vhost: Improve vhost_get_avail_idx() with
smp_rmb()
On Thu, Mar 28, 2024 at 10:21:49AM +1000, Gavin Shan wrote:
> All the callers of vhost_get_avail_idx() are concerned to the memory
> barrier, imposed by smp_rmb() to ensure the order of the available
> ring entry read and avail_idx read.
>
> Improve vhost_get_avail_idx() so that smp_rmb() is executed when
> the avail_idx is advanced. With it, the callers needn't to worry
> about the memory barrier.
>
> Suggested-by: Michael S. Tsirkin <mst@...hat.com>
> Signed-off-by: Gavin Shan <gshan@...hat.com>
Previous patches are ok. This one I feel needs more work -
first more code such as sanity checking should go into
this function, second there's actually a difference
between comparing to last_avail_idx and just comparing
to the previous value of avail_idx.
I will pick patches 1-2 and post a cleanup on top so you can
take a look, ok?
> ---
> drivers/vhost/vhost.c | 75 +++++++++++++++----------------------------
> 1 file changed, 26 insertions(+), 49 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 32686c79c41d..e6882f4f6ce2 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1290,10 +1290,28 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)
> mutex_unlock(&d->vqs[i]->mutex);
> }
>
> -static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
> - __virtio16 *idx)
> +static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
> {
> - return vhost_get_avail(vq, *idx, &vq->avail->idx);
> + __virtio16 avail_idx;
> + int r;
> +
> + r = vhost_get_avail(vq, avail_idx, &vq->avail->idx);
> + if (unlikely(r)) {
> + vq_err(vq, "Failed to access avail idx at %p\n",
> + &vq->avail->idx);
> + return r;
> + }
> +
> + vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> + if (vq->avail_idx != vq->last_avail_idx) {
> + /* Ensure the available ring entry read happens
> + * before the avail_idx read when the avail_idx
> + * is advanced.
> + */
> + smp_rmb();
> + }
> +
> + return 0;
> }
>
> static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
> @@ -2499,7 +2517,6 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> struct vring_desc desc;
> unsigned int i, head, found = 0;
> u16 last_avail_idx;
> - __virtio16 avail_idx;
> __virtio16 ring_head;
> int ret, access;
>
> @@ -2507,12 +2524,8 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> last_avail_idx = vq->last_avail_idx;
>
> if (vq->avail_idx == vq->last_avail_idx) {
> - if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
> - vq_err(vq, "Failed to access avail idx at %p\n",
> - &vq->avail->idx);
> + if (unlikely(vhost_get_avail_idx(vq)))
> return -EFAULT;
> - }
> - vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>
> if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
> vq_err(vq, "Guest moved used index from %u to %u",
> @@ -2525,11 +2538,6 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> */
> if (vq->avail_idx == last_avail_idx)
> return vq->num;
> -
> - /* Only get avail ring entries after they have been
> - * exposed by guest.
> - */
> - smp_rmb();
> }
>
> /* Grab the next descriptor number they're advertising, and increment
> @@ -2790,35 +2798,19 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
> /* return true if we're sure that avaiable ring is empty */
> bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> {
> - __virtio16 avail_idx;
> - int r;
> -
> if (vq->avail_idx != vq->last_avail_idx)
> return false;
>
> - r = vhost_get_avail_idx(vq, &avail_idx);
> - if (unlikely(r))
> - return false;
> -
> - vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> - if (vq->avail_idx != vq->last_avail_idx) {
> - /* Since we have updated avail_idx, the following
> - * call to vhost_get_vq_desc() will read available
> - * ring entries. Make sure that read happens after
> - * the avail_idx read.
> - */
> - smp_rmb();
> + if (unlikely(vhost_get_avail_idx(vq)))
> return false;
> - }
>
> - return true;
> + return vq->avail_idx == vq->last_avail_idx;
> }
> EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
>
> /* OK, now we need to know about added descriptors. */
> bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> {
> - __virtio16 avail_idx;
> int r;
>
> if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
> @@ -2842,25 +2834,10 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> /* They could have slipped one in as we were doing that: make
> * sure it's written, then check again. */
> smp_mb();
> - r = vhost_get_avail_idx(vq, &avail_idx);
> - if (r) {
> - vq_err(vq, "Failed to check avail idx at %p: %d\n",
> - &vq->avail->idx, r);
> + if (unlikely(vhost_get_avail_idx(vq)))
> return false;
> - }
> -
> - vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> - if (vq->avail_idx != vq->last_avail_idx) {
> - /* Since we have updated avail_idx, the following
> - * call to vhost_get_vq_desc() will read available
> - * ring entries. Make sure that read happens after
> - * the avail_idx read.
> - */
> - smp_rmb();
> - return true;
> - }
>
> - return false;
> + return vq->avail_idx != vq->last_avail_idx;
> }
> EXPORT_SYMBOL_GPL(vhost_enable_notify);
>
> --
> 2.44.0
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