On Fri, Aug 21, 2026 at 19:44:06 +0000, bd730c5053df9efb via Users wrote:
> Hi all!
>
> I have a slackware64-15.0 linux host running libvirt-11.10.0 with qemu 10.2.2
> and a windows XP guest. I'm trying to have the gustes COM1 port redirected to
> the hosts /dev/ttyUSB0 device. I have not found a direct way of doing this in
> virt-manager since the only options for a serial device are "Output to a
> file", "Pseudo TTY" or a "UNIX socket". I have managed to modify the domains
> XML adding the following
> > <commandline xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
> > <arg value='-chardev'/>
> > <arg value='serial,path=/dev/ttyUSB0,id=hostserial0'/>
> > <arg value='-device'/>
> > <arg value='pci-serial,chardev=hostserial0'/>
> > </commandline>
Use of qemu:commandline is at your own risk because libvirt can't know
what host resources the VM needs access for.
>
> When I start the VM with this I get an error that says
> > Could not open '/dev/ttyUSB0': Operation not permitted
So this is not surprising, because ...
>
> I checked the device's permissions which are u+rw,g+rw and qemu is being
> executed with root permissions, I added o+rw to /dev/ttyUSB0 just in case but
> got the same error. I also tried with /dev/tty0 but got the same error.
... libvirt creates a private /dev/ filesystem (via a mount namespace)
that is populated *only* by devices that the XML requests.
As qemu:commandline directly manipulates the commandline libvirt doesn't
know that you need access to /dev/ttyUSB0.
Said that there is no reason to do qemu:commandline backdoor as host
character devices are directly supported:
<serial type='dev'>
<source path='/dev/ttyUSB0'/>
<target port='0'/>
</serial>
https://www.libvirt.org/formatdomain.html#host-device-proxy
