linux-kernel - Re: [RFC PATCH V4 4/5] cpuidle: driver for xen

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Message-ID: <20110324120522.GB29294@dumpdata.com>
Date:	Thu, 24 Mar 2011 08:05:22 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Len Brown <lenb@...nel.org>
Cc:	Trinabh Gupta <trinabh@...ux.vnet.ibm.com>, arjan@...ux.intel.com,
	peterz@...radead.org, suresh.b.siddha@...el.com,
	benh@...nel.crashing.org, venki@...gle.com, ak@...ux.intel.com,
	linux-kernel@...r.kernel.org, sfr@...b.auug.org.au,
	xen-devel@...ts.xensource.com
Subject: Re: [RFC PATCH V4 4/5] cpuidle: driver for xen

On Thu, Mar 24, 2011 at 03:18:14AM -0400, Len Brown wrote:
> Is a CONFIG_XEN kernel supposed to use just HLT in idle?

For right now..
> 
> xen_arch_setup() does this:
> 
>         pm_idle = default_idle;
>         boot_option_idle_override = IDLE_HALT;
> 
> which has that effect.  I guess this makes sense b/c the
> CONFIG_XEN kernel is Dom0 and the real C-sates are done
> by the hypervisor?

Correct. There are some patches that make the C-states 
be visible in the Linux kernel, but that hasn't been ported
over yet.

> 
> Would the same CONFIG_XEN kernel binary ever not
> run xen_arch_setup(), run on raw hardware, and want

ever not? I am not sure of the question, so let me state:
The Linux kernel if compiled with CONFIG_XEN, and if run on
native hardware, would _never_ run 'xen_arch_setup()'*. It would
run the normal, native type setup.

> to use idle states other than HLT?
> 

*: It could if you really really wanted. You would need to change
the GRUB2 to inject some extra data in the 'sub_hardware' flag to be
the Xen specific.
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