Re: [Xen-devel] [PATCH 0 of 5] Make checkpointing backwards compatible with older kernels

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 12/1/07 10:03 pm, "Brendan Cully" <brendan@xxxxxxxxx> wrote:

> This set of patches makes checkpointing backwards compatible with
> older kernels. Checkpointable kernels now create a xenstore entry
> called "control/feature-resumable" which xc save -c checks before
> attempting to checkpoint a domain.
> 
> It also writes a new SIF_RESUMABLE flag into the suspend record,
> which xc_linux_restore uses to decide whether it is safe to set the
> hypercall return value.

A single unified new elfnote would probably be better. We would like a new
libxc function to pull image metadata (inc. Elfnotes) out of a kernel image
and provide a comma-separated key-value list, turned into a dictionary for
Python consumption. This would then be persisted by xend (e.g., be added to
save/restore stream format) and can be used to decide whether 'xm save -c'
is okay, and would allow info to be passed to xc_save/xc_restore about
whether suspend is resumable. We already have another intended use for this
mechanism, which is to provide a pte mask to use to decide whether or not a
not-present pte contains an MFN (needing conversion to PFN on xc_save).
Unfortunately although this method is cleanest there is some tedious
plumbing to be done and I'm not sure how to represent the Elf notes in a
save image format (perhaps it can be merged into the image sxpr).

Yes -- returning 1 on resume (rather than restore) will allow new guest
kernels to run properly on old Xen and tools which do not understand resume,
so that would be the better way round.

 -- Keir


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