This seems too crazy to work, but it does:
$ nbdkit memory 1G
$ nbdfuse mem nbd://localhost &
[1] 1053075
$ ll mem
-rw-rw-rw-. 1 rjones rjones 1073741824 May 17 18:31 mem
Now boot qemu with that memory as the backing RAM:
$ qemu-system-x86_64 -m 1024 \
-object memory-backend-file,id=pc.ram,size=1024M,mem-path=/var/tmp/mem,share=on \
-machine memory-backend=pc.ram \
-drive file=fedora-36.img,if=virtio,format=raw
It works! You can even dump the RAM over a second NBD connection and grep for strings which appear on the screen (or passwords etc):
$ nbdcopy nbd://localhost - | strings | grep 'There was 1 failed'
There was 1 failed login attempt since the last successful login.
Of course this isn’t very useful on its own, it’s just an awkward way to use a sparse RAM disk as guest RAM, but nbdkit has plenty of other plugins that might be useful here. How about remote RAM? You’ll need a very fast network.
Richard WM Jones 