Re: [PATCH V3 00/22] Live Update [restart] : code replication

[Steven Sistare]

On 6/3/2021 3:36 PM, Dr. David Alan Gilbert wrote:
> * Steven Sistare (steven.sistare@oracle.com) wrote:
>> On 5/24/2021 6:39 AM, Dr. David Alan Gilbert wrote:
>>> * Steven Sistare (steven.sistare@oracle.com) wrote:
>>>> On 5/20/2021 9:13 AM, Dr. David Alan Gilbert wrote:
>>>>> On the 'restart' branch of questions; can you explain,
>>>>> other than the passing of the fd's, why the outgoing side of
>>>>> qemu's 'migrate exec:' doesn't work for you?
>>>>
>>>> I'm not sure what I should describe.  Can you be more specific?
>>>> Do you mean: can we add the cpr specific bits to the migrate exec code?
>>>
>>> Yes; if possible I'd prefer to just keep the one exec mechanism.
>>> It has an advantage of letting you specify the new command line; that
>>> avoids the problems I'd pointed out with needing to change the command
>>> line if a hotplug had happened.  It also means we only need one chunk of
>>> exec code.
>>
>> [...]
> 
> I'm not quite sure what I want in the incoming there, but that is
> already the source execing the destination qemu - although I think we'd
> probably need to check if that's actually via an intermediary.
> 
>> We could shoehorn cpr restart into the migrate exec path by defining a new 
>> migration 
>> capability that the client would set before issuing the migrate command.  
>> However, the
>> implementation code would be sprinkled with conditionals to suppress 
>> migrate-only bits
>> and call cpr-only bits.  IMO that would be less maintainable than having a 
>> separate
>> cprsave function.  Currently cprsave does not duplicate any migration 
>> functionality.
>> cprsave calls qemu_save_device_state() which is used by xen.
> 
> To me it feels like cprsave in particular is replicating more code. 

In the attached file I annotated lines of code that have some overlap
with migration code actions.  They include vm control, global_state_store,
and vmstate save, and cover 18 lines of 78 total.  I did not include the
body of qf_file_open because it is also called by xen.

The migration code adds capabilities, parameters, state, status, info,
precopy, postcopy, dirty bitmap, events, notifiers, 6 channel types,
blockers, pause+resume, return path, request-reply commands, throttling, colo,
blocks, phases, iteration, and threading, implemented by 20000+ lines of code.
To me it seems wrong to throw cpr into that mix to avoid adding tens of lines 
of similar code.

- Steve

cprsave.txt
Description: Text document