Subject: Re: [PATCH net-next v11 01/12] virtio_pci: Remove supported_cap size build assert

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 Nov 2025 02:38:44 -0500

On Tue, Nov 18, 2025 at 08:38:51AM -0600, Daniel Jurgens wrote:
> The cap ID list can be more than 64 bits. Remove the build assert. Also
> remove caching of the supported caps, it wasn't used.
> 
> Signed-off-by: Daniel Jurgens <[email-masked]>
> Reviewed-by: Parav Pandit <[email-masked]>
> Reviewed-by: Xuan Zhuo <[email-masked]>
> 
> ---
> v4: New patch for V4
> v5:
>    - support_caps -> supported_caps (Alok Tiwari)
>    - removed unused variable (test robot)
> ---
>  drivers/virtio/virtio_pci_common.h | 1 -
>  drivers/virtio/virtio_pci_modern.c | 8 +-------
>  2 files changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
> index 8cd01de27baf..fc26e035e7a6 100644
> --- a/drivers/virtio/virtio_pci_common.h
> +++ b/drivers/virtio/virtio_pci_common.h
> @@ -48,7 +48,6 @@ struct virtio_pci_admin_vq {
>  	/* Protects virtqueue access. */
>  	spinlock_t lock;
>  	u64 supported_cmds;
> -	u64 supported_caps;
>  	u8 max_dev_parts_objects;
>  	struct ida dev_parts_ida;
>  	/* Name of the admin queue: avq.$vq_index. */
> diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
> index dd0e65f71d41..ff11de5b3d69 100644
> --- a/drivers/virtio/virtio_pci_modern.c
> +++ b/drivers/virtio/virtio_pci_modern.c
> @@ -304,7 +304,6 @@ virtio_pci_admin_cmd_dev_parts_objects_enable(struct virtio_device *virtio_dev)
>  
>  static void virtio_pci_admin_cmd_cap_init(struct virtio_device *virtio_dev)
>  {
> -	struct virtio_pci_device *vp_dev = to_vp_device(virtio_dev);
>  	struct virtio_admin_cmd_query_cap_id_result *data;
>  	struct virtio_admin_cmd cmd = {};
>  	struct scatterlist result_sg;
> @@ -323,12 +322,7 @@ static void virtio_pci_admin_cmd_cap_init(struct virtio_device *virtio_dev)
>  	if (ret)
>  		goto end;
>  
> -	/* Max number of caps fits into a single u64 */
> -	BUILD_BUG_ON(sizeof(data->supported_caps) > sizeof(u64));
> -
> -	vp_dev->admin_vq.supported_caps = le64_to_cpu(data->supported_caps[0]);
> -
> -	if (!(vp_dev->admin_vq.supported_caps & (1 << VIRTIO_DEV_PARTS_CAP)))
> +	if (!(le64_to_cpu(data->supported_caps[0]) & (1 << VIRTIO_DEV_PARTS_CAP)))
>  		goto end;

It's ok but a better way is

data->supported_caps[0] & cpu_to_le64(1 << VIRTIO_DEV_PARTS_CAP)

giving the compiler a chance to do the byte swap at compile time
on BE.

>  	virtio_pci_admin_cmd_dev_parts_objects_enable(virtio_dev);
> -- 
> 2.50.1