Re: [PATCH 1/2] block/blkio: fix opening virtio-blk drivers

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jul 24, 2023 at 05:46:10PM +0200, Stefano Garzarella wrote:
> libblkio 1.3.0 added support of "fd" property for virtio-blk-vhost-vdpa
> driver. In QEMU, starting from commit cad2ccc395 ("block/blkio: use
> qemu_open() to support fd passing for virtio-blk") we are using
> `blkio_get_int(..., "fd")` to check if the "fd" property is supported
> for all the virtio-blk-* driver.
> 
> Unfortunately that property is also available for those driver that do
> not support it, such as virtio-blk-vhost-user. Indeed now QEMU is
> failing if used with virtio-blk-vhost-user in this way:
> 
>    -blockdev node-name=drive0,driver=virtio-blk-vhost-user,path=vhost-user-blk.sock,cache.direct=on: Could not open 'vhost-user-blk.sock': No such device or address
> 
> So, `blkio_get_int()` is not enough to check whether the driver supports
> the `fd` property or not. This is because the virito-blk common libblkio
> driver only checks whether or not `fd` is set during `blkio_connect()`
> and fails for those transports that do not support it (all except
> vhost-vdpa for now).
> 
> So for now let's also check that the driver is virtio-blk-vhost-vdpa,
> since that's the only one that supports it.

What happens when more virtio-blk-* libblkio drivers gain support for
`fd`? I think we'll be back to the same problem because QEMU will be
unable to distinguish between old and new libraries.

How about retrying with `path` if opening with `fd` fails?

> 
> Fixes: cad2ccc395 ("block/blkio: use qemu_open() to support fd passing for virtio-blk")
> Reported-by: Qing Wang <qinwang@redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  block/blkio.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blkio.c b/block/blkio.c
> index 1798648134..ca1149042a 100644
> --- a/block/blkio.c
> +++ b/block/blkio.c
> @@ -662,6 +662,7 @@ static int blkio_virtio_blk_common_open(BlockDriverState *bs,
>          QDict *options, int flags, Error **errp)
>  {
>      const char *path = qdict_get_try_str(options, "path");
> +    const char *blkio_driver = bs->drv->protocol_name;
>      BDRVBlkioState *s = bs->opaque;
>      bool fd_supported = false;
>      int fd, ret;
> @@ -676,7 +677,8 @@ static int blkio_virtio_blk_common_open(BlockDriverState *bs,
>          return -EINVAL;
>      }
>  
> -    if (blkio_get_int(s->blkio, "fd", &fd) == 0) {
> +    if (strcmp(blkio_driver, "virtio-blk-vhost-vdpa") == 0 &&
> +        blkio_get_int(s->blkio, "fd", &fd) == 0) {
>          fd_supported = true;
>      }
>  
> -- 
> 2.41.0
> 

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