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@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
---
 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

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@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.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

On 3/28/24 19:31, Michael S. Tsirkin wrote:
> 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@redhat.com>
>> Signed-off-by: Gavin Shan <gshan@redhat.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?
> 

Thanks, Michael. It's fine to me.

Thanks,
Gavin

Hi Michael,

On 3/30/24 19:02, Gavin Shan wrote:
> On 3/28/24 19:31, Michael S. Tsirkin wrote:
>> 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@redhat.com>
>>> Signed-off-by: Gavin Shan <gshan@redhat.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?
>>
> 
> Thanks, Michael. It's fine to me.
> 

A kindly ping.

If it's ok to you, could you please merge PATCH[1-2]? Our downstream
9.4 need the fixes, especially for NVidia's grace-hopper and grace-grace
platforms.

For PATCH[3], I also can help with the improvement if you don't have time
for it. Please let me know.

Thanks,
Gavin

On Mon, Apr 08, 2024 at 02:15:24PM +1000, Gavin Shan wrote:
> Hi Michael,
> 
> On 3/30/24 19:02, Gavin Shan wrote:
> > On 3/28/24 19:31, Michael S. Tsirkin wrote:
> > > 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@redhat.com>
> > > > Signed-off-by: Gavin Shan <gshan@redhat.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?
> > > 
> > 
> > Thanks, Michael. It's fine to me.
> > 
> 
> A kindly ping.
> 
> If it's ok to you, could you please merge PATCH[1-2]? Our downstream
> 9.4 need the fixes, especially for NVidia's grace-hopper and grace-grace
> platforms.
> 
> For PATCH[3], I also can help with the improvement if you don't have time
> for it. Please let me know.
> 
> Thanks,
> Gavin

1-2 are upstream go ahead and post the cleanup.

-- 
MST

On 4/23/24 06:46, Michael S. Tsirkin wrote:
> On Mon, Apr 08, 2024 at 02:15:24PM +1000, Gavin Shan wrote:
>> On 3/30/24 19:02, Gavin Shan wrote:
>>> On 3/28/24 19:31, Michael S. Tsirkin wrote:
>>>> 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@redhat.com>
>>>>> Signed-off-by: Gavin Shan <gshan@redhat.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?
>>>>
>>>
>>> Thanks, Michael. It's fine to me.
>>>
>>
>> A kindly ping.
>>
>> If it's ok to you, could you please merge PATCH[1-2]? Our downstream
>> 9.4 need the fixes, especially for NVidia's grace-hopper and grace-grace
>> platforms.
>>
>> For PATCH[3], I also can help with the improvement if you don't have time
>> for it. Please let me know.
>>
> 
> 1-2 are upstream go ahead and post the cleanup.
> 

Michael, a cleanup series has been sent for review.

https://lore.kernel.org/virtualization/20240423032407.262329-1-gshan@redhat.com/T/#t

Thanks,
Gavin

On Mon, Apr 08, 2024 at 02:15:24PM +1000, Gavin Shan wrote:
> Hi Michael,
> 
> On 3/30/24 19:02, Gavin Shan wrote:
> > On 3/28/24 19:31, Michael S. Tsirkin wrote:
> > > 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@redhat.com>
> > > > Signed-off-by: Gavin Shan <gshan@redhat.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?
> > > 
> > 
> > Thanks, Michael. It's fine to me.
> > 
> 
> A kindly ping.
> 
> If it's ok to you, could you please merge PATCH[1-2]? Our downstream
> 9.4 need the fixes, especially for NVidia's grace-hopper and grace-grace
> platforms.
> 
> For PATCH[3], I also can help with the improvement if you don't have time
> for it. Please let me know.
> 
> Thanks,
> Gavin

The thing to do is basically diff with the patch I wrote :)
We can also do a bit more cleanups on top of *that*, like unifying
error handling.

-- 
MST

On Mon, Apr 08, 2024 at 02:15:24PM +1000, Gavin Shan wrote:
> Hi Michael,
> 
> On 3/30/24 19:02, Gavin Shan wrote:
> > On 3/28/24 19:31, Michael S. Tsirkin wrote:
> > > 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@redhat.com>
> > > > Signed-off-by: Gavin Shan <gshan@redhat.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?
> > > 
> > 
> > Thanks, Michael. It's fine to me.
> > 
> 
> A kindly ping.
> 
> If it's ok to you, could you please merge PATCH[1-2]? Our downstream
> 9.4 need the fixes, especially for NVidia's grace-hopper and grace-grace
> platforms.

Yes - in the next rc hopefully.

> For PATCH[3], I also can help with the improvement if you don't have time
> for it. Please let me know.
> 
> Thanks,
> Gavin


That would be great.

-- 
MST

On Thu, Mar 28, 2024 at 8:22 AM Gavin Shan <gshan@redhat.com> 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@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks