Re: [Xen-devel] [PATCH 1/4] docs: Clarify the expected behaviour of zero length records

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/07/16 11:35, David Vrabel wrote:
> On 25/07/16 11:25, Andrew Cooper wrote:
>> On 25/07/16 11:21, David Vrabel wrote:
>>> On 21/07/16 18:17, Andrew Cooper wrote:
>>>> The sending side shouldn't send any variable sized records which end up 
>>>> having
>>>> zero content, but the receiving side will need to tolerate such records for
>>>> compatibility purposes.
>>>>
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
>>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>> ---
>>>>  docs/specs/libxc-migration-stream.pandoc | 16 +++++++++++++++-
>>>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/docs/specs/libxc-migration-stream.pandoc 
>>>> b/docs/specs/libxc-migration-stream.pandoc
>>>> index 31eba10..a90bc5d 100644
>>>> --- a/docs/specs/libxc-migration-stream.pandoc
>>>> +++ b/docs/specs/libxc-migration-stream.pandoc
>>>> @@ -3,7 +3,7 @@
>>>>    Andrew Cooper <<andrew.cooper3@xxxxxxxxxx>>
>>>>    Wen Congyang <<wency@xxxxxxxxxxxxxx>>
>>>>    Yang Hongyang <<hongyang.yang@xxxxxxxxxxxx>>
>>>> -% Revision 1
>>>> +% Revision 2
>>>>  
>>>>  Introduction
>>>>  ============
>>>> @@ -631,6 +631,10 @@ The set of valid records depends on the guest 
>>>> architecture and type.  No
>>>>  assumptions should be made about the ordering or interleaving of
>>>>  independent records.  Record dependencies are noted below.
>>>>  
>>>> +Some records have an exactly specified size.  Some records have variable 
>>>> size
>>>> +depending on their content.  A record with variable size which ends up 
>>>> being
>>>> +zero should be omitted entirely from the stream by the sending side.
>>> I disagree. I think the stream should include the records with the empty
>>> content.  This gives better consistency and does not require changes to
>>> the stream.
>> There are already some which are properly omitted, like the vcpu records
>> for offline vcpus.
>>
>> There is no point having empty records; omitting them is an optimisation
>> which we absolutely shouldn't preclude.
> The optimization doesn't matter since these records are so tiny.
>
> I've expanded on why these should be included in another reply.

We already omit most zero-length records.  That ship has already sailed.

This bug only presents itself because of two hypercalls in Xen returning
different size characteristics.  We already checked for zero first, and
fail to check for zero the second time.

~Andrew

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