Re: [Xen-devel] [PATCH] tools/migrate: Fix regression when migrating from older version of Xen

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Thu, 2013-07-11 at 12:19 +0100, Andrew Cooper wrote:
> On 11/07/13 10:55, Ian Campbell wrote:
> > On Wed, 2013-07-10 at 20:19 +0100, Andrew Cooper wrote:
> >> Commit 00a4b65f8534c9e6521eab2e6ce796ae36037774 Sep 7 2010
> >>   "libxc: provide notification of final checkpoint to restore end"
> >> broke migration from any version of Xen using tools from prior to that 
> >> commit
> >>
> >> Older tools have no idea about an XC_SAVE_ID_LAST_CHECKPOINT, causing newer
> >> tools xc_domain_restore() to start reading the qemu save record, as
> >> ctx->last_checkpoint is 0.
> > Can the receive not distinguish the case where it is receiving a
> > checkpoint stream from a regular one? Either via some property of the
> > stream or the parameters with which it was called?
> >
> > ctx->last_checkpoint should (be made to) only matter if you are doing
> > check pointing and I think we can mandate that checking pointing is only
> > supported between like versions of Xen. TBH it's a bit odd to send the
> > LAST_CHECKPOINT in anon-checkpoint stream anyway, but what's done is
> > done.
> >
> > Ian.
> 
> Sending LAST_CHECKPOINT is the only thing which has allowed normal
> migration to work since this regression.
> 
> I cant find any way of distinguishing a normal vs checkpointed migrate
> from the stream alone.  The save_callback->checkpoint function pointer
> may or may not be filled in by a toolstack, but is completely
> out-of-band as far as the migration stream is concerned.

I suppose your new last_generation-aware flag is similar to the "is a
checkpoint restore" lag which I was thinking of, but it somewhat easier
for the toolstack to know that it is receiving a checkpoint vs. normal
migration compared with magically knowing the version on the other end
of the connection.

Ian.


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