Re: [PATCH 1/2] block/blkio: fix opening virtio-blk drivers

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Jul 26, 2023 at 09:26:45AM +0200, Stefano Garzarella wrote:
> 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?

No. Checking for -EINVAL and then retrying with `path` is what I had in
mind.

> 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?

I prefer retrying on -EINVAL because even if libblkio 1.3.1 is released
today, we don't have control over when it becomes available in distros.

Stefan

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