From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932401AbcERQAM (ORCPT ); Wed, 18 May 2016 12:00:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:53141 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932076AbcERQAK (ORCPT ); Wed, 18 May 2016 12:00:10 -0400 Subject: Re: [PATCH] xen: add steal_clock support on x86 To: Boris Ostrovsky , David Vrabel , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org References: <1463573758-11441-1-git-send-email-jgross@suse.com> <573C804F.6020708@oracle.com> <573C81D1.9040309@suse.com> <573C896F.4090303@oracle.com> <573C8D4A.1000005@suse.com> <573C8E16.5040409@citrix.com> <573C8FEA.2070407@oracle.com> Cc: sstabellini@kernel.org From: Juergen Gross Message-ID: <573C9188.7010303@suse.com> Date: Wed, 18 May 2016 18:00:08 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.0 MIME-Version: 1.0 In-Reply-To: <573C8FEA.2070407@oracle.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/05/16 17:53, Boris Ostrovsky wrote: > On 05/18/2016 11:45 AM, David Vrabel wrote: >> On 18/05/16 16:42, Juergen Gross wrote: >>> On 18/05/16 17:25, Boris Ostrovsky wrote: >>>> On 05/18/2016 10:53 AM, Juergen Gross wrote: >>>>> On 18/05/16 16:46, Boris Ostrovsky wrote: >>>>>> On 05/18/2016 08:15 AM, Juergen Gross wrote: >>>>>>> } >>>>>>> >>>>>>> +void __init xen_time_setup_guest(void) >>>>>>> +{ >>>>>>> + pv_time_ops.steal_clock = xen_steal_clock; >>>>>>> + >>>>>>> + static_key_slow_inc(¶virt_steal_enabled); >>>>>>> + /* >>>>>>> + * We can't set paravirt_steal_rq_enabled as this would require the >>>>>>> + * capability to read another cpu's runstate info. >>>>>>> + */ >>>>>>> +} >>>>>> Won't we be accounting for stolen cycles twice now --- once from >>>>>> steal_account_process_tick()->steal_clock() and second time from >>>>>> do_stolen_accounting()? >>>>> Uuh, yes. >>>>> >>>>> I guess I should rip do_stolen_accounting() out, too? >>>> I don't think PARAVIRT_TIME_ACCOUNTING is always selected for Xen. If >>> This is easy to accomplish. :-) > > > I looked at KVM code (PARAVIRT_TIME_ACCOUNTING is not selected there > neither) and in their case that's presumably because stealing accounting > is a CPUID bit, i.e. it might not be supported. In Xen case we always > have this interface. So they added it later and the default is to keep the old behavior. >>>> that's indeed the case then we should ifndef do_stolen_accounting(). Or >>>> maybe check for paravirt_steal_enabled. >>> Is this really a sensible thing to do? There is a mechanism used by KVM >>> to do the stolen accounting. I think we should use it instead of having >>> a second implementation doing the same thing in case the generic one >>> isn't enabled. >> I agree. >> >> Although I don't think selecting PARAVIRT_TIME_ACC' is necessary -- I >> don't think it's essential (or is it?). > > Looks like it's useful only if paravirt_steal_rq_enabled, which we don't > support yet. I think the patch is still useful. It is reducing code size and it is removing arch-specific Xen-hack(s). With the patch Xen's solution for arm and x86 is common and the same as for KVM. Adding paravirt_steal_rq_enabled later will be much easier as only one function needs substantial modification. Juergen