On Tue, Jul 25, 2023 at 04:00:38PM -0400, Stefan Hajnoczi wrote:
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.
If we release a v1.3.1 version of libblkio with
https://gitlab.com/libblkio/libblkio/-/merge_requests/208
we can set a minimum requirement in QEMU and use blkio_set_fd() here.
How about retrying with `path` if opening with `fd` fails?
IIUC the only way is to check if blkio_connect() will fail with -EINVAL,
that can also be generated by other issues, then retry forcing `path`.
Do you see other ways?
The code wouldn't be great, but we could do it for now and then when
we release a new version of libblkio, do the revert and use
blkio_set_int(fd) to see if it's supported or not.
I don't know if it is worth it, or if it is better to merge this,
release libblkio v1.3.1 and force the minimum requirement.
WDYT?
Thanks,
Stefano
