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

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Message-id: <alpine.LFD.2.02.1103250315180.32565@x980>
Date:	Fri, 25 Mar 2011 03:19:25 -0400 (EDT)
From:	Len Brown <lenb@...nel.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
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.

The Xen Dom0 kernel will trap into the hypervisor
whenever it does a HLT or an MWAIT, yes?

What is the benefit of having Dom0 decided between
C-states that it can't actually enter?

What is the mechanism by which those C-states are
made visible to Dom0, and how are those states
related to the states that are supported on
the bare iron?

> > Would the same CONFIG_XEN kernel binary ever not
> > run xen_arch_setup(), run on raw hardware, and want
> > to use idle states other than HLT?

> 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.

Thanks, that clarifies.
-Len Brown, Intel Open Source Technolgy Center

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