LXC GPU Access
Giving a LXC guest GPU access allows you to use a GPU in a guest while it is still being available to the host machine. This is a big advantage over virtual machines where only a single host or guest can have access to a GPU at one time. Even better, multipel LXC guests can share a GPU from the host at the same time.
Determine Device Major/Minor Numbers
To allow a container access to the device you'll have to know the devices major/minor numbers. This can be found easily enough by running ls -l in /dev/. As an example to pass through the integated UHD 630 GPU from an Core i7 8700k you would first list the devices where are created under /dev/dri.
root@blackbox:~# ls -l /dev/dri
total 0
drwxr-xr-x 2 root root 80 May 12 21:54 by-path
crw-rw---- 1 root video 226, 0 May 12 21:54 card0
crw-rw---- 1 root render 226, 128 May 12 21:54 renderD128
From that you can see the major device number is 226 and the minors are 0 and 128.
Provide LXC Access
In the configuration file you'd then add lines to allow the LXC guest access to that device and then also bind mount the devices from the host into the guest. In the example above since both devices share the same major number it is possible to use a shorthand notation of 226:* to represent all minor numbers with major number 226.
# /etc/pve/lxc/*.conf
+ lxc.cgroup.devices.allow: c 226:* rwm
+ lxc.mount.entry: /dev/dri/card0 dev/dri/card0 none bind,optional,create=file,mode=0666
+ lxc.mount.entry: /dev/dri/renderD128 dev/dri/renderD128 none bind,optional,create=file
Allow unprivileged Containers Access
In the example above we saw that card0 and renderD128 are both owned by root and have their groups set to video and render. Because the unprivilged part of LXC works by mapping the UIDs (user id) and GIDs (group id) in the LXC guest namespace to a high range of ids on host, it is necessary to create a custom mapping for that namespace that maps those groups in the LXC guest namespace to the host groups while leaving the rest unchanged so you don't lose the added security of running an unprivilged container.
First you need to give root permission to map the group IDs. You can look in /etc/group to find the GIDs of those groups, but in this example video = 44 and render = 108 on our host system. You should add the following lines that allow root to map those groups to a new GID.
# /etc/subgid
+ root:44:1
+ root:108:1
Then you'll need to create the ID mappings. Since you're just dealing with groups mapping the UIDs can be done in a single line as shown on the first line. It can be read as "remap 65,536 of the LXC guest namespaces UIDs from 0 through 65,536 to a range in the host starting at 100,000. You can tell this relates to UIDs because of the u denoting users. It wasn't necessary to edit /etc/subuid because that file already gives root permission to perform this mapping.
You have to do the same thing for groups which is the same concept but slightly more verbose. In this example when looking at /etc/group in the LXC guest it shows that video and render have GIDs of 44 and 106. Although you'll use g to denote GIDs everything else is the same except it is necessary to ensure the custom mappings cover the whole range of GIDs so it requires more lines. The only tricky part is the second to last line that shows mapping the LXC guest namespace GID for render (106) to the host GID for render (108) because the groups have different GIDs.
# /etc/pve/lxc/*.conf
lxc.cgroup.devices.allow: c 226:* rwm
lxc.mount.entry: /dev/dri/card0 dev/dri/card0 none bind,optional,create=file,mode=0666
lxc.mount.entry: /dev/dri/renderD128 dev/dri/renderD128 none bind,optional,create=file
+ lxc.idmap: u 0 100000 65536
+ lxc.idmap: g 0 100000 44
+ lxc.idmap: g 44 44 1
+ lxc.idmap: g 45 100045 61
+ lxc.idmap: g 106 108 1
+ lxc.idmap: g 107 100107 65429
Add root to Groups
Because root's UID and GID in the LXC guest's namespace isn't mapped to root on the host to access the devices you'll have to add any users to the groups video and render. As an example to give root in our LXC guest's namespace access to the devices you would simply add root to the video and render group.
usermod --append --groups video,render root