linux-kernel - RE: [Xen-devel] [PATCH 1/3] PAD helper for native and paravirt platform

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Date:	Thu, 23 Feb 2012 13:26:53 +0000
From:	"Liu, Jinsong" <jinsong.liu@...el.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"lenb@...nel.org" <lenb@...nel.org>,
	"Brown, Len" <len.brown@...el.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"keir.xen@...il.com" <keir.xen@...il.com>,
	Jan Beulich <JBeulich@...e.com>,
	"Li, Shaohua" <shaohua.li@...el.com>
Subject: RE: [Xen-devel] [PATCH 1/3] PAD helper for native and paravirt
 platform

Konrad Rzeszutek Wilk wrote:
> On Wed, Feb 22, 2012 at 05:02:59PM +0000, Liu, Jinsong wrote:
>> Konrad Rzeszutek Wilk wrote:
>>> On Tue, Feb 21, 2012 at 05:49:58AM +0000, Liu, Jinsong wrote:
>>>> Konrad Rzeszutek Wilk wrote:
>>>>>>>>> +struct pv_pad_ops {
>>>>>>>>> +	int (*acpi_pad_init)(void);
>>>>>>>>> +	void (*acpi_pad_exit)(void);
>>>>>>>>> +};
>>>>>>>>> +
>>>>>>> 
>>>>>>> Looking at this a bit closer I am not sure why you choose the
>>>>>>> paravirt interface for this? There is another one - the x86 that
>>>>>>> could have been choosen. Or introduce a new one that is
>>>>>>> specific to ACPI. 
>>>>>>> 
>>>>>>> I am curious - what was the reason for using the paravirt
>>>>>>> interface? I understand it does get the job done, but it seems a
>>>>>>> bit overkill when something simple could have been used?
>>>>>>> 
>>>>>> 
>>>>>> It uses paravirt interface to avoid some code like 'xen_...' in
>>>>>> native code path (acpi_pad.c).
>>>>>> I'm not quite sure what does 'x86' here mean? Adding 2 fields
>>>>>> (acpi_pad_init/exit) in arch/x86/xen/enlighten.c --> xen_cpu_ops?
>>>>>> seems it's much simpler.
>>>>> 
>>>>> arch/x86/include/asm/x86_init.h
>>>>> 
>>>>> But before you go that way let me ask you another question - can
>>>>> ACPI PAD be used on ARM or IA64? If so, wouldn't this fail
>>>>> compilation as this pvops structure is not defined on IA64?
>>>> 
>>>> Ideally ACPI PAD is not bound to some arch, so IMO it could be used
>>>> at least on IA64 (through currently no real PAD on IA64 platform as
>>>> far as I know). However, in native acpi_pad implementation, it
>>>> indeed depends on X86 for reason like mwait.
>>>> So for xen acpi_pad, I think it's OK to choose x86, defining an
>>>> acpi_pad_ops at x86_init.c which would be overwritten when xen
>>>> init. 
>>> 
>>> OK, or in osl.c. We need Len to chime in here as I can see this
>>> expanding in the future.
>>>> 
>>>> Another choice is to define config ACPI_PROCESSOR_AGGREGATOR as
>>>> 'bool', which would disable native acpi_pad module.
>>> 
>>> Ewww. No.
>> 
>> I'm OK with x86_init approach, but advantage of 'config
>> ACPI_PROCESSOR_AGGREGATOR as bool' would get rid of X86/IA64/...
>> arch issue for xen (at least from coding view), through it need
>> disable native acpi_pad module (IMO acpi_pad module has not strong
>> reason to must be so). Have a re-consider of this approach? :-)   
> 
> But it is a compile option right? We wantone kernel that can do both
> baremetal and Xen.

Sorry, I didn't speak clear my approach.
The approach can enable kernel run both baremetal and Xen platform, and not bind acpi pad logic to x86.
Send 2 patches as RFC.

Thanks
Jinsong--
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