Re: device hotplug & file handles

[Eric Blake <eblake@redhat.com>]

On 5/7/20 9:49 AM, Gerd Hoffmann wrote:
>    Hi,
> 
> For usb device pass-through (aka -device usb-host) it would be very
> useful to pass file handles from libvirt to qemu.  The workflow would
> change from ...
> 
>    (1) libvirt enables access to /dev/usb/$bus/$dev
>    (2) libvirt passes $bus + $dev (using hostbus + hostaddr properties)
>        to qemu.
>    (3) qemu opens /dev/usb/$bus/$dev
> 
> ... to ...
> 
>    (1) libvirt opens /dev/usb/$bus/$dev
>    (2) libvirt passes filehandle to qemu.
> 
> Question is how can we pass the file descriptor best?  My idea would be
> to simply add an fd property to usb-host:
> 
>   * Coldplug would be "-device usb-host,fd=<nr>" (cmd line).
>   * Hotplug would be "device_add usb-host,fd=<getfd-name>" (monitor).
> 
> Will that work from libvirt point of view?
> Or does anyone have an better idea?

Qemu already has -add-fd (both a CLI version, and a QMP version when a 
Unix socket can pass fds), at which point any existing interface that 
uses qemu_open() will understand the magic syntax /dev/fdset/NNN to 
refer to the existing fd previously passed in via -add-fd.  Libvirt is 
already able to use this feature for some cases (for example, see 
src/qemu/qemu_command.c:qemuBuildChrChardevFileStr).  So all that 
remains is making sure -device usb-host uses qemu_open(), and if it 
didn't already do so, also making sure libvirt can find a way to 
introspect when usb-host started supporting fdset usage.

Or put another way, let's use the generic fd mechanism that qemu already 
supports, rather than inventing yet another syntax.

> 
> thanks,
>    Gerd
> 
> PS: background: https://bugzilla.redhat.com/show_bug.cgi?id=1595525
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org