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

[Stefano Garzarella]

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