Re: device hotplug & file handles
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