[Qemu-devel] Tracking memory dirtying in QEMU

[Anthony Liguori]

Howdy,

I've been working on migration for QEMU and have run into a snag.  I've 
got a non-live migration patch that works quite happily[1].  I modified 
the save/restore code to not seek at all, and then basically pipe a save 
over a pipe to a subprocess (usually, ssh).

Conceptually, adding support for live migration is really easy.  All I 
think I need to do is extend the current code, to have a pre-save hook 
that is activated before the VM is stopped.  This hook will be called 
until it says it's done and then the rest of the save/load handlers are 
invoked.  At first, I'm just going to do a pre-save handler for RAM 
which should significantly reduce the amount of down time.  I think the 
only other device we'll have to handle specially is the VGA memory but 
I'm happy to ignore that for now.

So, all I really need is to be able to track which pages are dirtied.  I 
also need the a method to reset the dirty map.

I started looking at adding another map like phys_ram_dirty.  That seems 
to work for some of the IO_MEM_RAM pages, but not all.  My initial 
thought is that all memory operations should go through one of the 
st[bwl]_phys functions but that doesn't seem to be the case.

Can anyone provide me with some advice on how to do this?  Am I right in 
assuming that all IO will go through some function?

[1] http://hg.codemonkey.ws/qemu-pq/?f=758c26c82f52;file=qemu-migration.diff

Thanks,

Anthony Liguori