[2/2] block/blkio: add more comments on the fd passing handling

Commit Message

Stefano Garzarella Aug. 1, 2023, 4:03 p.m. UTC

As Hanna pointed out, it is not clear in the code why qemu_open()
can fail, and why blkio_set_int("fd") is not enough to discover
the `fd` property support.

Let's fix them by adding more details in the code comments.

Suggested-by: Hanna Czenczek <[e-mail]>
Signed-off-by: Stefano Garzarella <[e-mail]>
---
 block/blkio.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

Comments

Hanna Czenczek Aug. 2, 2023, 12:02 p.m. UTC | #1

On 01.08.23 18:03, Stefano Garzarella wrote:
> As Hanna pointed out, it is not clear in the code why qemu_open()
> can fail, and why blkio_set_int("fd") is not enough to discover
> the `fd` property support.
>
> Let's fix them by adding more details in the code comments.
>
> Suggested-by: Hanna Czenczek <[e-mail]>
> Signed-off-by: Stefano Garzarella <[e-mail]>
> ---
>   block/blkio.c | 15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)

Thanks!

Reviewed-by: Hanna Czenczek <[e-mail]>

Patch

diff --git a/block/blkio.c b/block/blkio.c
index 2d53a865e7..848b8189d0 100644
--- a/block/blkio.c
+++ b/block/blkio.c
@@ -713,6 +713,12 @@  static int blkio_virtio_blk_connect(BlockDriverState *bs, QDict *options,
          */
         fd = qemu_open(path, O_RDWR, NULL);
         if (fd < 0) {
+            /*
+             * qemu_open() can fail if the user specifies a path that is not
+             * a file or device, for example in the case of Unix Domain Socket
+             * for the virtio-blk-vhost-user driver. In such cases let's have
+             * libblkio open the path directly.
+             */
             fd_supported = false;
         } else {
             ret = blkio_set_int(s->blkio, "fd", fd);
@@ -734,9 +740,12 @@  static int blkio_virtio_blk_connect(BlockDriverState *bs, QDict *options,
 
     ret = blkio_connect(s->blkio);
     /*
-     * If the libblkio driver doesn't support the `fd` property, blkio_connect()
-     * will fail with -EINVAL. So let's try calling blkio_connect() again by
-     * directly setting `path`.
+     * Before https://gitlab.com/libblkio/libblkio/-/merge_requests/208
+     * (libblkio <= v1.3.0), setting the `fd` property is not enough to check
+     * whether the driver supports the `fd` property or not. In that case,
+     * blkio_connect() will fail with -EINVAL.
+     * So let's try calling blkio_connect() again by directly setting `path`
+     * to cover this scenario.
      */
     if (fd_supported && ret == -EINVAL) {
         fd_supported = false;