Re: [Xen-devel] VM checkpoint image format

[Prateek Sharma <prateeks@xxxxxxxxxxxx>]

On Sun, 22 Dec 2013, Andrew Cooper wrote:

> On 21/12/2013 16:31, Prateek Sharma wrote:
> > Hello all,
> >     Is the VM-save image format specified anywhere? (the file written
> > to by xc_domain_save). I am looking for what fields are saved in what
> > order.
> >     Or is the xc_domain_save function itself the specification? Can I
> > change the order of fields saved (thus changing the on-disk image
> > format) and expect no breakages? Ofcourse, I will need to change the
> > xc_domain_restore function as well.
> >
> > Thanks!
> > --Prateek
> There is no documentation that I am aware of, other than the code
> itself, and the code is a rats nest.  The format itself is quite lacking
> in terms of extensibility, and makes some assumptions which are not
> actually accurate.
>
> In my copious free time (of which I have none), or when XenServer
> decided that it actually wants migration which works in combination with
> ballooning (or any of the other more interesting memory modes), I have a
> plan to make a "migration protocol v2" which shall be formally specified
> in a document, and have ample room for forward extensibility in a
> backward compatible manner.
>
>
> As for changing the order of things.  So long as your xc_domain_save()
> and xc_domain_restore() have similar expectations of what order to
> put/find chunks of the stream, it should work fine.  It would help if
> you described what changes you wish to make.  We might be able to point
> you at the correct area.
>
> ~Andrew
I'm attempting to implement VM lazy-restore. To achieve that, it would be 
nice if page-table pages, vcpu state, GDT pages, etc are located _before_ 
the rest of the pages.
Is there some implicit dependency in the ordering of pages and the VCPU 
context?
Is there a way to access (and hence save) all the page-table pages of a 
guest?
Thanks!
--Prateek


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