device hotplug & file handles
Eric Blake
eblake at redhat.com
Thu May 7 17:05:54 UTC 2020
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
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