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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, Jul 10, 2013 at 8:19 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> 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.
>
> The failure looks like:
>   xc: error: Max batch size exceeded (1970103633). Giving up.
> where 1970103633 = 0x756d6551 = *(uint32_t*)"Qemu"
>
> Sadly, the simple fix of just setting ctx->last_checkpoint = 1 will cause an
> opposite function regresson for anyone using the current behaviour of
> save_callbacks->checkpoint().
>
> The only safe fix is to rely on the toolstack to provide this information.
>
> Passing 0 results in unchanged behaviour, while passing nonzero means "the
> other end of the migration stream does not know about
> XC_SAVE_ID_LAST_CHECKPOINT but is performing a normal migrate"
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Shouldn't there also be a way to actually use the flag?

 -George

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