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

[Steven Sistare]

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.

How/where would you specify a new command line?  Are you picturing the usual 
migration
setup where you start a second qemu with its own arguments, plus a 
migrate_incoming
option or command?  That does not work for live update restart; the old qemu 
must exec
the new qemu.  Or something else?

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.

- Steve