Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933618Ab2FBSHA (ORCPT ); Sat, 2 Jun 2012 14:07:00 -0400 Received: from e28smtp01.in.ibm.com ([122.248.162.1]:45266 "EHLO e28smtp01.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933556Ab2FBSG6 (ORCPT ); Sat, 2 Jun 2012 14:06:58 -0400 Message-ID: <4FCA5608.2010404@linux.vnet.ibm.com> Date: Sat, 02 Jun 2012 23:36:00 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120424 Thunderbird/12.0 MIME-Version: 1.0 To: Jan Beulich CC: Russell King , yong.zhang0@gmail.com, Jeremy Fitzhardinge , peterz@infradead.org, mingo@kernel.org, x86@kernel.org, Thomas Gleixner , akpm@linux-foundation.org, nikunj@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, vatsa@linux.vnet.ibm.com, virtualization@lists.linux-foundation.org, xen-devel@lists.xensource.com, Konrad Rzeszutek Wilk , Ingo Molnar , rusty@rustcorp.com.au, rjw@sisk.pl, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Keir Fraser , "H. Peter Anvin" Subject: Re: [Xen-devel] [PATCH 05/27] xen, cpu hotplug: Don't call cpu_bringup() in xen_play_dead() References: <20120601090952.31979.24799.stgit@srivatsabhat.in.ibm.com> <20120601091124.31979.91984.stgit@srivatsabhat.in.ibm.com> <4FC8D8E30200007800087D2B@nat28.tlf.novell.com> <4FC8DC19.4000007@linux.vnet.ibm.com> <4FC8FD9C0200007800087DF5@nat28.tlf.novell.com> In-Reply-To: <4FC8FD9C0200007800087DF5@nat28.tlf.novell.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit x-cbid: 12060218-4790-0000-0000-0000030E0D84 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2369 Lines: 60 On 06/01/2012 09:06 PM, Jan Beulich wrote: >>>> On 01.06.12 at 17:13, "Srivatsa S. Bhat" > wrote: >> On 06/01/2012 06:29 PM, Jan Beulich wrote: >> >>>>>> On 01.06.12 at 11:11, "Srivatsa S. Bhat" >>> 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. > Ok.. So, I would love to hear a confirmation about whether this patch (which removes cpu_bringup() in xen_play_dead()) will break things or it is good as is. If its not correct, then we can probably make __cpu_post_online() return an int, with the meaning: 0 => success, go ahead and call cpu_idle() non-zero => stop here, thanks for your services so far.. now leave the rest to me. So all other archs will return 0, Xen will return non-zero, and it will handle when to call cpu_idle() and when not to do so. Might sound a bit ugly, but I don't see much other option. Suggestions are appreciated! Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/