extract lines from bottom until regex match

This feels a bit silly, but:

$ tac file.txt |sed -e '/^virt-top/q' |tac
virt-top time  11:25:17 Host foo.example.com x86_64 32/32CPU 1200MHz 65501MB
   ID S RDRQ WRRQ RXBY TXBY %CPU %MEM   TIME    NAME
    1 R    0    0    0    0  0.6 12.0  96:02:53 instance-0000036f
    2 R    0    0    0    0  0.2 12.0  95:44:08 instance-00000372

GNU tac reverses the file (many non-GNU systems have tail -r instead), the sed picks lines until the first that starts with virt-top. You can add sed 1,2d or tail -n +3 to remove the headers.

Or in awk:

$ awk '/^virt-top/ { a = "" } { a = a $0 ORS } END {printf "%s", a}' file.txt 
virt-top time  11:25:17 Host foo.example.com x86_64 32/32CPU 1200MHz 65501MB
   ID S RDRQ WRRQ RXBY TXBY %CPU %MEM   TIME    NAME
    1 R    0    0    0    0  0.6 12.0  96:02:53 instance-0000036f
    2 R    0    0    0    0  0.2 12.0  95:44:08 instance-00000372

It just collects all the lines to a variable, and clears that variable on a line starting with virt-top.

If the file is very large, the tac+sed solution is bound to be faster since it only needs to read the tail end of the file while the awk solution reads the full file from the top.

With ed you can regex-search upward using ?pattern? in place of the usual /pattern/ (which searches from above the current position). So for example:

$ printf '%s\n' '?ID?+1,$p' q | ed -s file.txt
    1 R    0    0    0    0  0.6 12.0  96:02:53 instance-0000036f
    2 R    0    0    0    0  0.2 12.0  95:44:08 instance-00000372

If your input has a fixed number of blocks you could also do something like:

awk '/^virt-top/ && ++n == 2, 0' <your-file

To output the lines from the 2^nd occurrence of virt-top to the end of the file (0 meaning false, means the end of that first,last range is never found).