linux-kernel - Re: [Xen-devel] [PATCH 05/27] xen, cpu hotplug: Don't call cpu_bringup() in xen_play

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Date:	Fri, 01 Jun 2012 16:36:28 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Russell King" <linux@....linux.org.uk>, <yong.zhang0@...il.com>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>, <peterz@...radead.org>,
	<mingo@...nel.org>, <x86@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	<akpm@...ux-foundation.org>, <nikunj@...ux.vnet.ibm.com>,
	<paulmck@...ux.vnet.ibm.com>, <vatsa@...ux.vnet.ibm.com>,
	<virtualization@...ts.linux-foundation.org>,
	<xen-devel@...ts.xensource.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>, <rusty@...tcorp.com.au>,
	<rjw@...k.pl>, <linux-arch@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, "Keir Fraser" <keir@....org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 05/27] xen, cpu hotplug: Don't call
 cpu_bringup() in xen_play_dead()

>>> On 01.06.12 at 17:13, "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
wrote:
> On 06/01/2012 06:29 PM, Jan Beulich wrote:
> 
>>>>> On 01.06.12 at 11:11, "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
>> wrote:
>>> xen_play_dead calls cpu_bringup() which looks weird, because xen_play_dead()
>>> is invoked in the cpu down path, whereas cpu_bringup() (as the name 
>>> suggests) is useful in the cpu bringup path.
>> 
>> This might not be correct - the code as it is without this change is
>> safe even when the vCPU gets onlined back later by an external
>> entity (e.g. the Xen tool stack), and it would in that case resume
>> at the return point of the VCPUOP_down hypercall. That might
>> be a heritage from the original XenoLinux tree though, and be
>> meaningless in pv-ops context - Jeremy, Konrad?
>> 
>> Possibly it was bogus/unused even in that original tree - Keir?
>>
> 
> 
> Thanks for your comments Jan!
> 
> In case this change is wrong, the other method I had in mind was to call
> cpu_bringup_and_idle() in xen_play_dead(). (Even ARM does something similar,
> in the sense that it runs the cpu bringup code including cpu_idle(), in the
> cpu offline path, namely the cpu_die() function). Would that approach work
> for xen as well? If yes, then we wouldn't have any issues to convert xen to
> generic code.

No, that wouldn't work either afaict - the function is expected
to return.

Jan

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