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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/07/13 10:46, George Dunlap wrote:
> On Wed, Jul 10, 2013 at 8:19 PM, Andrew Cooper
> <andrew.cooper3@citrix.com> 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@citrix.com>
> Shouldn't there also be a way to actually use the flag?
>
>  -George

How do you mean? Plumb it up through to xl?

xl (in any non-experemental form) post-dates this regression.  xend
predates it, but I can't find any way of working out whether the far end
predates the regression.

~Andrew