and a flat sparsemem map, you're only looking atAt the moment my concern is 32-bit x86, which doesn't support vmemmap or sections smaller than 512MB because of the shortage of page flags bits.
~500k of overhead for the sparsemem storage. Less if you use vmemmap.
Yeah, I forgot that we didn't have vmemmap on x86-32. Ugh.
OK, here's another idea: Xen (and the balloon driver) already handle a
case where a guest boots up with 2GB of memory but only needs 1GB,
right? It will balloon the guest down to 1GB from 2GB.
Why don't we just have hotplug work that way? When we want to take a
guest from 1GB to 1GB+1 page (or whatever), we just hotplug the entire
section (512MB or 1GB or whatever), actually online the whole thing,
then make the balloon driver take it back to where it *should* be. That
way we're completely reusing existing components that have do be able to
handle this case anyway.
Yeah, this is suboptimal, an it has a possibility of fragmenting the
memory, but it will only be used for the x86-32 case.