Re: [PATCH v3 1/1] block/blkio: use qemu_open() to support fd passing for virtio-blk

[Jonathon Jongsma]

On 5/15/23 5:10 AM, Stefano Garzarella wrote:
> On Thu, May 11, 2023 at 11:03:22AM -0500, Jonathon Jongsma wrote:
> > On 5/11/23 4:15 AM, Stefano Garzarella wrote:
> > > The virtio-blk-vhost-vdpa driver in libblkio 1.3.0 supports the new
> > > 'fd' property. Let's expose this to the user, so the management layer
> > > can pass the file descriptor of an already opened vhost-vdpa character
> > > device. This is useful especially when the device can only be accessed
> > > with certain privileges.
> > >
> > > If the libblkio virtio-blk driver supports fd passing, let's always
> > > use qemu_open() to open the `path`, so we can handle fd passing
> > > from the management layer through the "/dev/fdset/N" special path.
> > >
> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > ---
> > >
> > > Notes:
> > >     v3:
> > >     - use qemu_open() on `path` to simplify libvirt code [Jonathon]
> >
> >
> > Thanks
> >
> > The one drawback now is that it doesn't seem possible for libvirt to 
> > introspect whether or not qemu supports passing an fd to the driver or 
> > not.
>
> Yep, this was because the libblkio library did not support this new way.
>
> > When I was writing my initial patch (before I realized that it was 
> > missing fd-passing), I just checked for the existence of the 
> > virtio-blk-vhost-vdpa device. But we actually need to know both that 
> > this device exists and supports fd passing.
>
> Yep, this was one of the advantages of using the new `fd` parameter.
> Can't libvirt handle the later failure?

Not very well. libvirt tries to provide useful errors to the user. So 
for example if the qemu executable doesn't support a device, we would 
want to provide an error indicating that the device is not supported 
rather than a possibly-inscrutable qemu error.

For example, in this scenario, we would want an error such as:

error: unsupported configuration: vhostvdpa disk is not supported with 
this QEMU binary

Instead of:

error: internal error: qemu unexpectedly closed the monitor: 
2023-05-16T15:17:36.666129Z qemu-system-x86_64: -blockdev 
{"driver":"virtio-blk-vhost-vdpa","path":"/dev/fdset/0","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}: 
blkio_connect failed: Failed to connect to vDPA device: Input/output error

And we can only do that if we can determine that the binary has the 
proper support for fds.

> > As far as I can tell, versions 7.2.0 and 8.0.0 include this device but 
> > won't accept fds.
>
> Right.
>
> How do you suggest to proceed?

I need some way to determine that the particular qemu binary can accept 
a /dev/fdset/ path for vdpa block devices. libvirt uses a variety of 
methods to determine capabilities for a given qemu binary, including 
querying the qmp schema, commands, object types, specific device/object 
properties, etc. For example, right now I can determine (via querying 
the qmp schema) whether virtio-blk-vhost-vdpa is a valid type for the 
blockdev-add command by querying the qmp schema. I need something more 
than that but I'm not sure how to do it without introducing a separate 
'fd' parameter. Any ideas?

Thanks,
Jonathon