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Re: [Xen-devel] [PATCH V1 1/2] Xen acpi memory hotplug driver

Liu, Jinsong wrote:
> Konrad Rzeszutek Wilk wrote:
>> On Thu, Dec 06, 2012 at 04:27:36AM +0000, Liu, Jinsong wrote:
>>>>>>> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig index
>>>>>>> 126d8ce..abd0396 100644 --- a/drivers/xen/Kconfig
>>>>>>> +++ b/drivers/xen/Kconfig
>>>>>>> @@ -206,4 +206,15 @@ config XEN_MCE_LOG
>>>>>>>           Allow kernel fetching MCE error from Xen platform and
>>>>>>>           converting it into Linux mcelog format for mcelog tools
>>>>>>> +       bool "Xen ACPI memory hotplug"
>>>>>> There should be a way to make this a module.
>>>>> I have some concerns to make it a module:
>>>>> 1. xen and native memhotplug driver both work as module, while we
>>>>> need early load xen driver. 
>>>>> 2. if possible, a xen stub driver may solve load sequence issue,
>>>>>   but it may involve other issues * if xen driver load then
>>>>> unload, native driver may have chance to load successfully;
>>>> The stub driver would still "occupy" the ACPI bus for the memory
>>>> hotplug PnP, so I think this would not be a problem.
>>> I'm not quite clear your mean here, do you mean it has
>>> 1. xen_stub driver + xen_memhoplug driver, then xen_strub driver
>>> unload and entirely replaced by xen_memhotplug driver, or
>>> 2. xen_stub driver (w/ stub ops) + xen_memhotplug ops (not driver),
>>> then xen_stub driver keep occupying but its stub ops later replaced
>>> by xen_memhotplug ops?
>> #2
>>> If in way #1, it has risk that native driver may load (if xen
>>> driver unload). If in way #2, xen_memhotplug ops lose the chance to
>>> probe/add/bind existed memory devices (since it's done when driver
>>> registerred). 
>> Could the stub driver have a queue of events?
> If so, why not do 'real' add ops (like our patch did, to build-in xen
> memory hotplug logic)? 
> I'm not quite clear your purpose of insisting module -- what's
> advantage of module you prefer? 
>>>>>   * if xen driver load --> unload --> load again, then it will
>>>>> lose hotplug notification during unload period;
>>>> Sure. But I think we can do it with this driver? After all the
>>>> function of it is to just tell the firmware to turn on/off sockets
>>>> - and if we miss one notification we won't take advantage of the
>>>> power savings - but we can do that later on.
>>> Not only inform firmware.
>>> Hotplug notify callback will invoke acpi_bus_add -> ... ->
>>> implicitly invoke drv->ops.add method to add the hotadded memory
>>> device.
>> Gotcha.
> ? So it will lose the notification and no way to add the new memory
> device in the future. 
> Xen memory hotplug logic consist of 2 parts:
> 1) driver logic (.add/.remove etc)
> 2) notification install/callback logic
> If you want to use 'xen_stub driver + .add/.remove ops', then
> notification install/callback logic would implement with xen_stub
> driver (means in build-in part, otherwise it would lose notification
> when the ops unload) --> but that would make xen_stub in big build-in
> size.    

How about
* build-in part: xen_stub driver (stub .add to record what matched cpu devices) 
+ notification install/callback;
* module part: .add/.remove ops;
w/ it, native driver has no chance to load and no hotplug event lose, and 
approximately 1/3 code is build-in and 2/3 are module.

I think it will work but I'm not quite sure, at least we can have a try/test?


>>>>>   * if xen driver load --> unload --> load again, then it will
>>>>> re-add all memory devices, but the handle for 'booting memory
>>>>> device' and 'hotplug memory device' are different while we have no
>>>>> way to distinguish these 2 kind of devices.
>>>> Wouldn't the stub driver hold onto that?
>>> Same question as comment #1. Do you mean it has a xen_stub driver
>>> (w/ stub ops) and a xen_memhotplug ops?
>> Correct.
>>>>> IMHO I think to make xen hotplug logic as module may involves
>>>>> unexpected result. Is there any obvious advantages of doing so?
>>>>> after all we have provided config choice to user. Thoughts?
>>>> Yes, it becomes a module - which is what we want.
>>> What I meant here is, module will bring some unexpected issues for
>>> xen hotplug. We can provide user 'bool' config choice, let them
>>> decide to build-in or not, but not 'tristate' choice.
>> What would be involved in making it an tristate choice?
>>> Thanks
>>> Jinsong
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