VM checkpoint image format

VM checkpoint image format

[Prateek Sharma]

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

Re: VM checkpoint image format

[Andrew Cooper]

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

>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

Re: VM checkpoint image format

[Prateek Sharma]

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

Re: VM checkpoint image format

[Ian Campbell]

On Sun, 2013-12-22 at 16:45 -0500, Prateek Sharma wrote:
> Is there a way to access (and hence save) all the page-table pages of a 
> guest?
> 

The code in xc_domain_save.c:canonicalize_pagetable is doing something
along these lines.

Ian.

Re: VM checkpoint image format

[Ian Campbell]

On Sun, 2013-12-22 at 15:29 +0000, 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,

There is a big comment in tools/libxc/xg_save_restore.h which describes
the migration protocol. The save format is basically just a single pass
of that.

Ian