RE: udmabuf error with libvirt + QEMU

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Hi Daniel,

You're right , /dev/udmabuf is required for GTK display for GL support. I agree passing the device using qemu:arg breaks the security confinement of Libvirt. For our use-case we need to have the GL support, is there any way to specify the GTK display option in the Libvirt XML ? or any other alternatives?


-----Original Message-----
From: Daniel P. Berrangé <berrange@xxxxxxxxxx> 
Sent: Tuesday, February 1, 2022 8:11 PM
To: M, Shivakumar <shivakumar.m@xxxxxxxxx>
Cc: libvirt-users@xxxxxxxxxx
Subject: Re: udmabuf error with libvirt + QEMU

On Tue, Feb 01, 2022 at 02:27:55PM +0000, M, Shivakumar wrote:
> Hi,
> We are seeing an issue with udambuf, where it says "open /dev/udmabuf: No such file or directory " even if the device exits. This issue particularly we are seeing with libvirt. When we run the QEMU args on the command line, everything works as expected.
> It seems to be some permission issue when we use the Libvirt, please help us on resolving this.

When libvirt launches QEMU it puts in place a number of strict security protections. Libvirt will grant access on a per-file basis to resources on the host that QEMU should be allowed to access based on the device configuration in the XML.

In your case though you're using command line passthrough:

>   <qemu:commandline>
>     <qemu:arg value="-device"/>
>     <qemu:arg value="virtio-vga,blob=true"/>
>     <qemu:arg value="-display"/>
>     <qemu:arg value="gtk,gl=on"/>
>     <qemu:env name="DISPLAY" value=":1.0"/>
>   </qemu:commandline>

This is totally opaque to libvirt and so libvirt won't be granting access to any resources needed by these args. I'm assuming /dev/udmabuf is needed by the GTK display for GL support, or something along those lines.

For further information about your options please consult this page:

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