Re: [Xen-devel] save image file format

[John Levon <levon@xxxxxxxxxxxxxxxxx>]

On Thu, Sep 14, 2006 at 02:04:40PM +0100, Ian Pratt wrote:

> The plan is to fix much of this post 3.0.3 while integrating HVM
> save/restore. Would be good to have your help on this.

Does this mean somebody is working on it already? I saw Jun's proposed
format changes briefly during his presentation and they seemed
inadequate in terms of fixing the problems I see in the current format.
But certainly since Jun has a vested interest I'd be happy to help come
up with a reasonable format.

> save/restore is quite different from a core, so I'm not sure its worth
> doing #5.

Can you explain why you think so? In particular what difficulties do you
expect with using the same format for both?

Both contain VCPU context(s), PFN tables and their associated page contents.
The places where they differ are essentially minor.

I think (from the Linux POV) getting ptrace support on save files for
nearly free is certainly worth it.

> For the sake of optimizing IO, keeping the data pages aligned is
> worthwhile (I also want to make support for RDMA during live relo easy
> as well).

There's no reason this can't be done.

> For PV guest live relo we have to transfer page type info, and
> there is some advantage to transferring this 'as we go' rather than in a
> big batch at the end.

Absolutely, as I mentioned, any format would need to have enough leeway
to support streaming (and, after all, page resends). It's perfectly
possible to have a slightly modified ELF-style format for this...

> > There also remains the question of supporting older Xens. I 
> > don't believe that anything other than migration 
> > back-compatibility is interesting in this case, and 
> > presumably this can be handled reasonably easily by a slight 
> > refactoring of xc_linux_{save,restore} (on the presumption 
> > that people will want to migrate both to and from such older 
> > dom0's), and falling back to compat mode when the new 
> > negotation API fails.
> 
> It's an interesting question whether we can get away with breaking old
> saved images. We've never guaranteed stability of this ABI, but it would
> be nice to retain backward compatibility if it's not too hard.

It's probably easy enough to do.

regards
john

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