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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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.

David

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