I'm using KUbuntu 20.04 on the Dell E6410. Supposedly it has GT218M [NVS 3100M] display adapter (at least, lshw says so). For a long time I've been using the nouveau driver (xserver-xorg-video-nouveau), but today I've noticed a webpage that causes an OS hang for me (when I open that webpage via either Firefox or Google Chrome, it reproducibly causes a full hang, even SysRq keys don't work; with the nvidia-340 driver this doesn't happen).
I would like to report this problem to nouveau. I see the bug reporting manual and specifically the hang diagnosis manual. But the problem is that I still doesn't understand how to get kernel messages: on the one hand – in the first manual they claim /var/log/dmesg isn't what they need, on the other hand – the output of the dmesg command seems to be cleared on reboot. The hang seems to be at least level-8 per their scale in the second manual. Their first suggestion is “mounting the partition, where system logs are written, with sync option”, but, although I can mount the root partition with the sync option before reproducing the hang, I don't know which file to get then (as they say /var/log/dmesg isn't what they need).