From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from va3outboundpool.messaging.microsoft.com (va3ehsobe003.messaging.microsoft.com [216.32.180.13]) (using TLSv1 with cipher AES128-SHA (128/128 bits)) (Client CN "mail.global.frontbridge.com", Issuer "Microsoft Secure Server Authority" (not verified)) by ozlabs.org (Postfix) with ESMTPS id D69CCB7033 for ; Wed, 18 Apr 2012 02:25:20 +1000 (EST) Message-ID: <4F8D995C.9030300@freescale.com> Date: Tue, 17 Apr 2012 11:25:00 -0500 From: Scott Wood MIME-Version: 1.0 To: Li Yang-R58472 Subject: Re: [PATCH v4 1/4] powerpc/85xx: add HOTPLUG_CPU support References: <1331889732-25240-1-git-send-email-chenhui.zhao@freescale.com> <4F8C78B3.1070002@freescale.com> <94F013E7935FF44C83EBE7784D62AD3F092D1098@039-SN2MPN1-022.039d.mgd.msft.net> In-Reply-To: <94F013E7935FF44C83EBE7784D62AD3F092D1098@039-SN2MPN1-022.039d.mgd.msft.net> Content-Type: text/plain; charset="ISO-8859-1" Cc: Wood Scott-B07421 , "linuxppc-dev@lists.ozlabs.org" , "linux-kernel@vger.kernel.org" , Zhao Chenhui-B35336 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 04/17/2012 04:51 AM, Li Yang-R58472 wrote: >>> struct smp_ops_t smp_85xx_ops = { >>> .kick_cpu = smp_85xx_kick_cpu, >>> -#ifdef CONFIG_KEXEC >>> +#ifdef CONFIG_HOTPLUG_CPU >>> + .cpu_disable = generic_cpu_disable, >>> + .cpu_die = generic_cpu_die, >>> +#endif >>> .give_timebase = smp_generic_give_timebase, >>> .take_timebase = smp_generic_take_timebase, >>> -#endif >>> }; >> >> We need to stop using smp_generic_give/take_timebase, not expand its use. >> This stuff breaks under hypervisors where timebase can't be written. It >> wasn't too bad before since we generally didn't enable CONFIG_KEXEC, but >> we're more likely to want CONFIG_HOTPLUG_CPU. > > I understand that the guest OS shouldn't change the real timebase. Cannot change it, and we've seen the tbsync code loop forever when it tries (since the changes aren't taking effect). > But no matter what timebase syncing method we are using, the timebase need to be changed anyway for certain features. That's why I said to do it the way U-Boot does it. > I think the better way should be trapping timebase modification in the hypervisor. It does trap. Currently we treat it as a no-op. The only reasonable alternative is to give the guest an exception. It is simply not allowed for a guest to modify the timebase -- we are not going to break the host's timebase sync. See the virtualized implementation note in section 9.2.1 of book III-E of Power ISA 2.06B: "In virtualized implementations, TBU and TBL are read-only." >> Do the timebase sync the way U-Boot does -- if you find the appropriate >> guts node in the device tree. > > That involves stopping timebase for a short time on all cores including the cores that are still online. Won't this be a potential issue? I don't think it's a big deal in the contexts where you'd be doing this -- at least not worse than the current situation. Just make sure that you don't reset the timebase to zero or otherwise make a core see the timebase go backward. -Scott