Re: [PATCH] block/blkio: add 'fd' option to virtio-blk-vhost-vdpa driver

[Stefan Hajnoczi]

On Tue, May 02, 2023 at 04:50:50PM +0200, Stefano Garzarella wrote:
> The virtio-blk-vhost-vdpa driver in libblkio 1.3.0 supports the new
> 'fd' property. Let's expose this to the user, so the management layer
> can pass the file descriptor of an already opened vhost-vdpa character
> device. This is useful especially when the device can only be accessed
> with certain privileges.
> 
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> 
> Notes:
>     As an alternative we could support passing `/dev/fdset/N` via 'path',
>     always opening the path with qemu_open() and passing the fd to the
>     libblkio driver.
>     I preferred to add a new parameter though, because the code is
>     simpler without changing how path works (alternatively we should check
>     first if fd is supported by the driver or not).
>     
>     What do you think?

I think the approach in this patch is fine.

>     
>     Thanks,
>     Stefano
> 
>  qapi/block-core.json |  6 +++++-
>  block/blkio.c        | 45 +++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 49 insertions(+), 2 deletions(-)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index b57978957f..9f70777d49 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -3841,10 +3841,14 @@
>  #
>  # @path: path to the vhost-vdpa character device.
>  #
> +# @fd: file descriptor of an already opened vhost-vdpa character device.
> +#      (Since 8.1)
> +#
>  # Since: 7.2
>  ##
>  { 'struct': 'BlockdevOptionsVirtioBlkVhostVdpa',
> -  'data': { 'path': 'str' },
> +  'data': { '*path': 'str',
> +            '*fd': 'str' },
>    'if': 'CONFIG_BLKIO' }
>  
>  ##
> diff --git a/block/blkio.c b/block/blkio.c
> index 0cdc99a729..98394b5745 100644
> --- a/block/blkio.c
> +++ b/block/blkio.c
> @@ -694,6 +694,49 @@ static int blkio_virtio_blk_common_open(BlockDriverState 
> *bs,
>      return 0;
>  }
>  
> +static int blkio_virtio_blk_vhost_vdpa_open(BlockDriverState *bs,
> +        QDict *options, int flags, Error **errp)
> +{
> +    const char *path = qdict_get_try_str(options, "path");
> +    const char *fd_str = qdict_get_try_str(options, "fd");
> +    BDRVBlkioState *s = bs->opaque;
> +    int ret;
> +
> +    if (path && fd_str) {
> +        error_setg(errp, "'path' and 'fd' options are mutually exclusive");
> +        return -EINVAL;
> +    }
> +
> +    if (!path && !fd_str) {
> +        error_setg(errp, "none of 'path' or 'fd' options was specified");
> +        return -EINVAL;
> +    }
> +
> +    if (path) {
> +        ret = blkio_set_str(s->blkio, "path", path);
> +        qdict_del(options, "path");
> +        if (ret < 0) {
> +            error_setg_errno(errp, -ret, "failed to set path: %s",
> +                             blkio_get_error_msg());
> +            return ret;
> +        }
> +    } else {
> +        ret = blkio_set_str(s->blkio, "fd", fd_str);

monitor_fd_param() is used by vhost-net, vhost-vsock, vhost-scsi, etc.

I think QEMU should parse the fd string and resolve it to a file
descriptor so the fd passing syntax matches the other vhost devices.

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