Re: [PATCH V3 11/16] migration: cpr-transfer mode

[Steven Sistare <steven.sistare@oracle.com>]

On 11/19/2024 4:48 PM, Peter Xu wrote:
> On Tue, Nov 19, 2024 at 04:41:07PM -0500, Steven Sistare wrote:
>> On 11/19/2024 4:29 PM, Peter Xu wrote:
>>> On Tue, Nov 19, 2024 at 04:03:08PM -0500, Steven Sistare wrote:
>>>> On 11/19/2024 3:51 PM, Peter Xu wrote:
>>>>> On Tue, Nov 19, 2024 at 03:32:55PM -0500, Steven Sistare wrote:
>>>>>> This begs the question, should we allow channels to be specified in hmp migrate
>>>>>> commands and for -incoming, in a very simple way?  Like with a prefix naming
>>>>>> the channel.  And eliminate the -cpr-uri argument. Examples:
>>>>>>
>>>>>> (qemu) migrate -d main:tcp:0:44444,cpr:unix:cpr.sock
>>>>>>
>>>>>> qemu -incoming main:tcp:0:44444,cpr:unix:cpr.sock
>>>>>> qemu -incoming main:defer,cpr:unix:cpr.sock
>>>>>
>>>>> IMHO keeping the old syntax working would still be nice to not break
>>>>> scripts.
>>>>
>>>> The channel tag would be optional, so backwards compatible.  Its unambiguous
>>>> as long as the channel names are not also protocol names.
>>>
>>> Ah that's ok then.  Or maybe use "="?
>>>
>>>     "main=XXX,cpr=XXX"
>>>
>>> Then if no "=" it's the old?
>>
>> Sure, that works.
>>
>>>>> I was thinking we could simply add one more parameter for taking
>>>>> cpr uri, like:
>>>>>
>>>>>        {
>>>>>            .name       = "migrate",
>>>>>            .args_type  = "detach:-d,resume:-r,uri:s,cpr:s?",
>>>>>            .params     = "[-d] [-r] uri [cpr_uri]",
>>>>>            .help       = "migrate to URI (using -d to not wait for completion)"
>>>>> 		      "\n\t\t\t -r to resume a paused postcopy migration",
>>>>> 		      "\n\t\t\t Setup cpr_uri to migrate with cpr-transfer",
>>>>>            .cmd        = hmp_migrate,
>>>>>        },
>>>>
>>>> That's fine.
>>>>
>>>> I do like the incoming syntax, though, instead of -cpr-uri.  What do you think?
>>>
>>> That'll definitely be lovely if possible, though would any monitor be alive
>>> at all before taking a cpr stream, with this series alone?  I thought you
>>> dropped the precreate, then QEMU isn't able to run the monitor loop until
>>> cpr-uri is loaded.
>>
>> No monitor or precreate changes.  I would parse -incoming, extract and use the cpr
>> channel early, and use the main channel later as usual.  It's just a different way of
>> specifying cpr-uri.  I like it because the specification language is more consistent,
>> referring to a "cpr channel" both on the outgoing and incoming side:
>>
>>    This mode requires a second migration channel named "cpr", included in
>>    the channel arguments of the migrate command on the outgoing side, and
>>    in the QEMU -incoming parameter on the incoming side.  The channel must
>>    be a type, such as unix socket, that supports SCM_RIGHTS.
> 
> Ah, that's ok at least to me.  I hope defer could still work (for Libvirt),
> though.  Probably something like main=defer,cpr=XXX.

Exactly.  defer is in my examples above.

- Steve