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

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Message-ID: <20120222182312.GE3132@phenom.dumpdata.com>
Date:	Wed, 22 Feb 2012 13:23:12 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	"Liu, Jinsong" <jinsong.liu@...el.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

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.
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