Yes, I considered that, but there are other memory regions that cannot be controlled
by the command line but which must be preserved, such as vram, bios, and rom. If vram
is not preserved, parts of the screen will be blank until the user performs some action
which refreshes the display. bios and rom should be preserved rather than re-recreated
with potentially different contents from the firmware images in the updated qemu package.
However, your comment reminds me that I must add a few lines of code to preserve the
memory-backend-memfd.
A new option specific to memory is the wrong way to do this. If a special mode must be specified when starting QEMU, you can make it a -machine option and block the QMP commands unless it's specified. Otherwise you can use "normal" migration to marshal and unmarshal across the update those memory regions that aren't backed by shared memory or memfd.
Also, because of the mess that vl.c had grown into, adding new "simple" options is going to be very very hard. In fact I am working on turning many options like -smp or -m into syntactic sugar for -machine; at some point I would like to (almost) forbid adding _any_ new option.
Paolo