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From: Gavin Shan <gshan@redhat.com>
To: virtualization@lists.linux.dev
Cc: linux-kernel@vger.kernel.org,
	mst@redhat.com,
	jasowang@redhat.com,
	shan.gavin@gmail.com
Subject: [PATCH v2 1/4] vhost: Improve vhost_get_avail_idx() with smp_rmb()
Date: Mon, 29 Apr 2024 20:13:57 +1000
Message-ID: <20240429101400.617007-2-gshan@redhat.com>
In-Reply-To: <20240429101400.617007-1-gshan@redhat.com>
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From: "Michael S. Tsirkin" <mst@redhat.com>

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.

No functional change intended.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[gshan: repainted vhost_get_avail_idx()]
Reviewed-by: Gavin Shan <gshan@redhat.com>
Acked-by: Will Deacon <will@kernel.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 */
+	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 (unlikely(r < 0))
+		return false;
 
-	return false;
+	return r;
 }
 EXPORT_SYMBOL_GPL(vhost_enable_notify);
 
-- 
2.44.0