Re: [PATCH 2/2] block/blkio: use blkio_set_int("fd") to check fd support

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jul 24, 2023 at 05:46:11PM +0200, Stefano Garzarella wrote:
> The way the virtio-blk driver is implemented in libblkio,
> it's much easier to use blkio_set_int() instead of blkio_get_int()
> and have it fail right away to see if `fd` is supported by the
> transport. See https://gitlab.com/libblkio/libblkio/-/merge_requests/208

The commit description is vague about what's going on here. My
understanding is:

  Setting the `fd` property fails with virtio-blk-* libblkio drivers
  that do not support fd passing since
  https://gitlab.com/libblkio/libblkio/-/merge_requests/208.

  Getting the `fd` property, on the other hand, always succeeds for
  virtio-blk-* libblkio drivers even when they don't support fd passing.

  This patch switches to setting the `fd` property because it is a
  better mechanism for probing fd passing support than getting the `fd`
  property.

Please update the commit description. Thanks!

> 
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  block/blkio.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blkio.c b/block/blkio.c
> index ca1149042a..719b19324b 100644
> --- a/block/blkio.c
> +++ b/block/blkio.c
> @@ -665,7 +665,7 @@ static int blkio_virtio_blk_common_open(BlockDriverState *bs,
>      const char *blkio_driver = bs->drv->protocol_name;
>      BDRVBlkioState *s = bs->opaque;
>      bool fd_supported = false;
> -    int fd, ret;
> +    int ret;
>  
>      if (!path) {
>          error_setg(errp, "missing 'path' option");
> @@ -678,7 +678,7 @@ static int blkio_virtio_blk_common_open(BlockDriverState *bs,
>      }
>  
>      if (strcmp(blkio_driver, "virtio-blk-vhost-vdpa") == 0 &&
> -        blkio_get_int(s->blkio, "fd", &fd) == 0) {
> +        blkio_set_int(s->blkio, "fd", -1) == 0) {
>          fd_supported = true;
>      }
>  
> @@ -688,7 +688,7 @@ static int blkio_virtio_blk_common_open(BlockDriverState *bs,
>       * layer through the "/dev/fdset/N" special path.
>       */
>      if (fd_supported) {
> -        int open_flags;
> +        int open_flags, fd;
>  
>          if (flags & BDRV_O_RDWR) {
>              open_flags = O_RDWR;
> -- 
> 2.41.0
> 

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