Re: [PATCH] virtio_ring: return an error pointer from vring_create_virtqueue()

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 05, 2026 at 08:34:25PM +0100, Stjepan Poljak wrote:
> vring_create_virtqueue() and vring_create_virtqueue_map() return NULL
> on any failure, so the error returned from vring_alloc_queue_split()
> is lost and turned into -ENOMEM. This is especially misleading if
> vring_alloc_queue_split() rejects a queue size with -EINVAL (i.e. if
> queue size is not a power of two):
> 
>   virtio_blk virtio0: Bad virtqueue length 100
>   virtio_blk virtio0: probe with driver virtio_blk failed with error -12
> 
> So the error points whoever debugs it at an allocation failure that
> never happened.
> 
> Return an ERR_PTR() instead: the error from vring_alloc_queue_split()
> or vring_alloc_queue_packed(), or -ENOMEM when allocating the
> virtqueue itself fails. Make the callers, virtio_pci (modern and
> legacy), virtio_mmio, virtio_vdpa, virtio_ccw and virtio_uml, check
> with IS_ERR() and pass the error on. The error path in virtio_ccw
> deletes a non-NULL vq, so clear vq there before taking it.
> 
> Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

not how you ae supposed to attribute LLM use.

> Signed-off-by: Stjepan Poljak <stjepan.poljak@gmail.com>


> ---
> Tested on x86_64 with a virtio-mmio virtio-blk device offering a
> 100-entry queue: the probe now fails with -22 instead of -12, and a
> 128-entry queue probes as before. The other transports are only
> compile-tested (x86_64, ARCH=um, s390 with clang, W=1).
> 
> No Fixes: tag, since the error has been -ENOMEM since 2010 and nothing
> regressed.
> 
> This conflicts trivially with patch 07/12 of Alexander Graf's "virtio:
> support devices that own their virtqueue memory" v2:
> https://lore.kernel.org/virtualization/20260818211425.91009-1-graf@amazon.com/
> ---
>  arch/um/drivers/virtio_uml.c       |  4 ++--
>  drivers/s390/virtio/virtio_ccw.c   |  5 +++--
>  drivers/virtio/virtio_mmio.c       |  4 ++--
>  drivers/virtio/virtio_pci_legacy.c |  4 ++--
>  drivers/virtio/virtio_pci_modern.c |  4 ++--
>  drivers/virtio/virtio_ring.c       | 12 +++++++-----
>  drivers/virtio/virtio_vdpa.c       |  4 ++--
>  include/linux/virtio_ring.h        |  6 +++++-
>  8 files changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/um/drivers/virtio_uml.c b/arch/um/drivers/virtio_uml.c
> index 881c34098a74..8b645523277f 100644
> --- a/arch/um/drivers/virtio_uml.c
> +++ b/arch/um/drivers/virtio_uml.c
> @@ -984,8 +984,8 @@ static struct virtqueue *vu_setup_vq(struct virtio_device *vdev,
>  
>  	vq = vring_create_virtqueue(index, num, PAGE_SIZE, vdev, true, true,
>  				    ctx, vu_notify, callback, info->name);
> -	if (!vq) {
> -		rc = -ENOMEM;
> +	if (IS_ERR(vq)) {
> +		rc = PTR_ERR(vq);
>  		goto error_create;
>  	}
>  	vq->priv = info;
> diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> index 552d77998012..c5a30f0a2bc2 100644
> --- a/drivers/s390/virtio/virtio_ccw.c
> +++ b/drivers/s390/virtio/virtio_ccw.c
> @@ -590,10 +590,11 @@ static struct virtqueue *virtio_ccw_setup_vq(struct virtio_device *vdev,
>  				    vdev, true, may_reduce, ctx,
>  				    notify, callback, name);
>  
> -	if (!vq) {
> +	if (IS_ERR(vq)) {
>  		/* For now, we fail if we can't get the requested size. */
>  		dev_warn(&vcdev->cdev->dev, "no vq\n");
> -		err = -ENOMEM;
> +		err = PTR_ERR(vq);
> +		vq = NULL;
>  		goto out_err;
>  	}
>  
> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> index e2ec0ec01d67..c403a025006e 100644
> --- a/drivers/virtio/virtio_mmio.c
> +++ b/drivers/virtio/virtio_mmio.c
> @@ -396,8 +396,8 @@ static struct virtqueue *vm_setup_vq(struct virtio_device *vdev, unsigned int in
>  	/* Create the vring */
>  	vq = vring_create_virtqueue(index, num, VIRTIO_MMIO_VRING_ALIGN, vdev,
>  				 true, true, ctx, notify, callback, name);
> -	if (!vq) {
> -		err = -ENOMEM;
> +	if (IS_ERR(vq)) {
> +		err = PTR_ERR(vq);
>  		goto error_new_virtqueue;
>  	}
>  
> diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
> index 8115aa39e01e..9d94fffff344 100644
> --- a/drivers/virtio/virtio_pci_legacy.c
> +++ b/drivers/virtio/virtio_pci_legacy.c
> @@ -130,8 +130,8 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
>  				    VIRTIO_PCI_VRING_ALIGN, &vp_dev->vdev,
>  				    true, false, ctx,
>  				    vp_notify, callback, name);
> -	if (!vq)
> -		return ERR_PTR(-ENOMEM);
> +	if (IS_ERR(vq))
> +		return vq;
>  
>  	vq->num_max = num;
>  
> diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
> index 922e2df027de..28ccb2130e27 100644
> --- a/drivers/virtio/virtio_pci_modern.c
> +++ b/drivers/virtio/virtio_pci_modern.c
> @@ -720,8 +720,8 @@ static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
>  				    SMP_CACHE_BYTES, &vp_dev->vdev,
>  				    true, true, ctx,
>  				    notify, callback, name);
> -	if (!vq)
> -		return ERR_PTR(-ENOMEM);
> +	if (IS_ERR(vq))
> +		return vq;
>  
>  	vq->num_max = num;
>  
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index f683a2d6a200..6dd4a7651bde 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1421,13 +1421,13 @@ static struct virtqueue *vring_create_virtqueue_split(
>  	err = vring_alloc_queue_split(&vring_split, vdev, num, vring_align,
>  				      may_reduce_num, map);
>  	if (err)
> -		return NULL;
> +		return ERR_PTR(err);
>  
>  	vq = __vring_new_virtqueue_split(index, &vring_split, vdev, weak_barriers,
>  				   context, notify, callback, name, map);
>  	if (!vq) {
>  		vring_free_split(&vring_split, vdev, map);
> -		return NULL;
> +		return ERR_PTR(-ENOMEM);
>  	}
>  
>  	to_vvq(vq)->we_own_ring = true;
> @@ -2671,15 +2671,17 @@ static struct virtqueue *vring_create_virtqueue_packed(
>  {
>  	struct vring_virtqueue_packed vring_packed = {};
>  	struct virtqueue *vq;
> +	int err;
>  
> -	if (vring_alloc_queue_packed(&vring_packed, vdev, num, map))
> -		return NULL;
> +	err = vring_alloc_queue_packed(&vring_packed, vdev, num, map);
> +	if (err)
> +		return ERR_PTR(err);
>  
>  	vq = __vring_new_virtqueue_packed(index, &vring_packed, vdev, weak_barriers,
>  					context, notify, callback, name, map);
>  	if (!vq) {
>  		vring_free_packed(&vring_packed, vdev, map);
> -		return NULL;
> +		return ERR_PTR(-ENOMEM);
>  	}
>  
>  	to_vvq(vq)->we_own_ring = true;
> diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
> index f9559b6b2e0b..c1b6134b2267 100644
> --- a/drivers/virtio/virtio_vdpa.c
> +++ b/drivers/virtio/virtio_vdpa.c
> @@ -208,8 +208,8 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
>  	vq = vring_create_virtqueue_map(index, max_num, align, vdev,
>  					true, may_reduce_num, ctx,
>  					notify, callback, name, map);
> -	if (!vq) {
> -		err = -ENOMEM;
> +	if (IS_ERR(vq)) {
> +		err = PTR_ERR(vq);
>  		goto error_new_virtqueue;
>  	}
>  
> diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> index c97a12c1cda3..c808ca7e8d58 100644
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -66,6 +66,9 @@ struct device;
>   * may_reduce_num is set, then this may allocate a smaller ring than
>   * expected.  The caller should query virtqueue_get_vring_size to learn
>   * the actual size of the ring.
> + *
> + * Returns the virtqueue or an ERR_PTR() on failure: -EINVAL if num is
> + * not a valid ring size, -ENOMEM if the ring could not be allocated.
>   */
>  struct virtqueue *vring_create_virtqueue(unsigned int index,
>  					 unsigned int num,
> @@ -80,7 +83,8 @@ struct virtqueue *vring_create_virtqueue(unsigned int index,
>  
>  /*
>   * Creates a virtqueue and allocates the descriptor ring with per
> - * virtqueue mapping operations.
> + * virtqueue mapping operations.  Returns the virtqueue or an ERR_PTR()
> + * on failure, like vring_create_virtqueue().
>   */
>  struct virtqueue *vring_create_virtqueue_map(unsigned int index,
>  					     unsigned int num,
> 
> base-commit: 1a577f9402c1780cc1e663f48d5dd54c02051b09
> -- 
> 2.53.0