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

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

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
> 

On Tue, Jul 25, 2023 at 04:05:38PM -0400, Stefan Hajnoczi wrote:
>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!

Ack, I'll update it.

Thanks,
Stefano

>
>>
>> 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
>>