I strongly suspect it would give you the content of the kernel log ring buffer (what "dmesg" alone gives you), but up until the time that userspace comes up, pr

I strongly suspect it would give you the content of the kernel log ring buffer (what "dmesg" alone gives you), but up until the time that userspace comes up, probably. A lot of distributions have something very similar in /var/log/dmesg or /var/log/boot or similar. They do this by just doing something like "dmesg > /var/log/dmesg" early at boot so that the boot messages are (hopefully) still in the ring buffer.