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Date: Mon, 29 Apr 2024 14:44:52 -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, shan.gavin@...il.com
Subject: Re: [PATCH v2 1/4] vhost: Improve vhost_get_avail_idx() with
smp_rmb()
On Mon, Apr 29, 2024 at 08:13:57PM +1000, Gavin Shan wrote:
> From: "Michael S. Tsirkin" <mst@...hat.com>
>
> All the callers of vhost_get_avail_idx() are concerned to the memory
*with* the memory barrier
> 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.
accessed, not advanced. guest advances it.
> With it, the callers needn't to worry
> about the memory barrier.
>
> No functional change intended.
I'd add:
As a side benefit, we also validate the index on all paths now, which
will hopefully help catch future errors earlier.
Note: current code is inconsistent in how it handles errors:
some places treat it as an empty ring, others - non empty.
This patch does not attempt to change the existing behaviour.
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> [gshan: repainted vhost_get_avail_idx()]
?repainted?
> Reviewed-by: Gavin Shan <gshan@...hat.com>
> Acked-by: Will Deacon <will@...nel.org>
> ---
> drivers/vhost/vhost.c | 106 +++++++++++++++++-------------------------
> 1 file changed, 42 insertions(+), 64 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8995730ce0bf..7aa623117aab 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1290,10 +1290,36 @@ 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 idx;
> + int r;
> +
> + r = vhost_get_avail(vq, idx, &vq->avail->idx);
> + if (unlikely(r < 0)) {
> + vq_err(vq, "Failed to access available index at %p (%d)\n",
> + &vq->avail->idx, r);
> + return r;
> + }
> +
> + /* Check it isn't doing very strange thing with available indexes */
> + vq->avail_idx = vhost16_to_cpu(vq, idx);
> + if (unlikely((u16)(vq->avail_idx - vq->last_avail_idx) > vq->num)) {
> + vq_err(vq, "Invalid available index change from %u to %u",
> + vq->last_avail_idx, vq->avail_idx);
> + return -EINVAL;
> + }
> +
> + /* We're done if there is nothing new */
> + if (vq->avail_idx == vq->last_avail_idx)
> + return 0;
> +
> + /*
> + * We updated vq->avail_idx so we need a memory barrier between
> + * the index read above and the caller reading avail ring entries.
> + */
> + smp_rmb();
> + return 1;
> }
>
> static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
> @@ -2498,38 +2524,17 @@ 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;
> + u16 last_avail_idx = vq->last_avail_idx;
> __virtio16 ring_head;
> int ret, access;
>
> - /* Check it isn't doing very strange things with descriptor numbers. */
> - 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);
> - 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 avail index from %u to %u",
> - last_avail_idx, vq->avail_idx);
> - return -EFAULT;
> - }
> + ret = vhost_get_avail_idx(vq);
> + if (unlikely(ret < 0))
> + return ret;
>
> - /* If there's nothing new since last we looked, return
> - * invalid.
> - */
> - if (vq->avail_idx == last_avail_idx)
> + if (!ret)
> 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 +2795,20 @@ 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();
> - return false;
> - }
> -
> - return true;
> + /* Treat error as non-empty here */
If you write the comment like that then put it before "return":
that is where you treat an error like this.
And I feel Note: is better in that the comment does not
explain all of what is going on, just an aspect of it.
> + r = vhost_get_avail_idx(vq);
> + return r == 0;
> }
> 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 +2832,13 @@ 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);
> - 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;
> - }
> + /* Treat error as empty here */
> + r = vhost_get_avail_idx(vq);
If you write the comment like that then put it before "return":
that is where you treat an error like this.
And I feel Note: is better in that the comment does not
explain all of what is going on, just an aspect of it.
> + if (unlikely(r < 0))
> + return false;
>
> - return false;
> + return r;
> }
> EXPORT_SYMBOL_GPL(vhost_enable_notify);
>
> --
> 2.44.0
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