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

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Date:	Fri, 25 Mar 2011 14:38:58 +0000
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Len Brown <lenb@...nel.org>, venki@...gle.com, ak@...ux.intel.com,
	suresh.b.siddha@...el.com, sfr@...b.auug.org.au,
	peterz@...radead.org, benh@...nel.crashing.org,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	arjan@...ux.intel.com, Trinabh Gupta <trinabh@...ux.vnet.ibm.com>
Subject: Re: [Xen-devel] Re: [RFC PATCH V4 4/5] cpuidle: driver for xen

On 03/24/2011 12:05 PM, Konrad Rzeszutek Wilk wrote:
> 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..

For always, I should think.

>> 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.

All we need is for the idle CPU to block in the hypervisor; a plain
"hlt" is always going to be sufficient (which is overridden as a pvop
into a sched_idle hypercall).

Xen will choose an appropriate power state for the physical cpus
depending on the overall busyness of the system (which any individual
virtual machine can't determine).

    J
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