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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/07/16 11:21, 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.

~Andrew

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