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Re: [libvirt] [PATCH 2/2] qemu_cgroup: allow access to /dev/dri/render*



On 20 May 2016 at 00:23, Daniel P. Berrange <berrange redhat com> wrote:
> On Thu, May 19, 2016 at 04:12:52PM +0200, Gerd Hoffmann wrote:
>>   Hi,
>>
>> > $ ls -lZ /dev/dri/
>> > total 0
>> > crw-rw----+ 1 root video system_u:object_r:dri_device_t:s0 226,   0 May 18
>> > 19:17 card0
>> > crw-------. 1 root video system_u:object_r:dri_device_t:s0 226,  64 May 18
>> > 19:17 controlD64
>> > crw-rw----+ 1 root video system_u:object_r:dri_device_t:s0 226, 128 May 18
>> > 19:17 renderD128
>> >
>> > qemu itself loops over /dev/dri, grabs the first matching renderD* that it can
>> > open, then does its magic. Some questions I have in general:
>> >
>> > - is there only ever one render node? or one per video card?
>>
>> One per video card.
>
> Is there a way to tell QEMU which video card to use ? If so we need to
> somehow represent this in libvirt.

We should probably add support for using an explicit path as the backing
for a particular virtio-gpu device. At the moment I think we just open the
first which may or may not be a great decision.

>
>> > - is it okay to use the same node for multiple VMs simultaneously?
>>
>> Yes.
>
> Presumably they're going to compete for execution time and potentially
> VRAM at least ? I assume they have 100% security isolation from each
> other though.  IOW, permissioning is really just there to prevent a
> rogue processes from doing denial of service  on the GPU resources,
> rather than actively compromising other users of the GPU ?

Securing 3D accelerated VM access to 100% is unlikely to ever be possible,
the GPU hardware just doesn't support this in some cases, later GPU
hardware is a lot better, but there will always be DoS and possible info
leaks through a GPU. I don't think VMware or anyone else do much
different here. What using a render node does is blocks you from deliberately
/accidentally accessing other users buffers using the defined API the
old drm API had a global namespace you could stumble through for
shared buffers.

>> > Maybe the long term fix is to have libvirt pass in a pre-opened fd for
>> > qemu:///system, since I don't know if it would be acceptable to chown
>> > qemu:qemu on the render node, but maybe we use setfacl instead.
>>
>> chown isn't a good idea I think.  But doesn't use libvirt setfacl anyway
>> for simliar things (i.e. /dev/bus/usb/... for usb pass-through) ?
>
> No, we exclusively switch access to QEMU only.
>
> Obviously the DRI stuff is different as we expected the host OS to
> have continued concurrent use of the video card.

chowning wouldn't be acceptable, adding an acl for qemu:qemu
would be fine though.

Dave.


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