Growing a para-virtualized domain beyond its initial allocation?

[John Byrne <john.l.byrne@hp.com>]

I didn't get any responses on xen-users, so I'll try xen-devel:

This is somewhere between a user and a developer question, depending on
the answers. My tests were on the latest SuSE drop of xen: 3.0.2_09434-0
on i386.

I want to start a domain with a small amount of memory, but have the
potential to give it more if it needs it. (Perhaps after migrating to a
machine with more memory available.)

A simple "xm mem-set" doesn't work like I want, Linux never sees the
memory. Having looked at the balloon driver code, it would seem that
specifying "mem=256M" on the kernel command line and "memory = 128M" and
"maxmem = 256M" in the config file should do the right things, but in
practice it doesn't work. When I try the "xm mem-set 4 256",
/proc/xen/balloon in the guest reports that I have hit the Xen hard
limit. (I haven't yet figured out where this is coming from.)

So, am I on the right track, but there is a bug? Should I be going about
this another way? Is it hopeless? Assuming this works for
para-virtualized domains, is there any hope for doing the same with a
hvm-domain. (I'd guess not, but I'm supposed to ask.)

Thanks,

John Byrne