From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2001:470:1f0b:db:abcd:42:0:1]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 2655E1A0064 for ; Fri, 27 Feb 2015 00:31:25 +1100 (AEDT) Message-ID: <54EF2025.80404@linutronix.de> Date: Thu, 26 Feb 2015 14:31:17 +0100 From: Sebastian Andrzej Siewior MIME-Version: 1.0 To: Paolo Bonzini , Scott Wood Subject: Re: [PATCH 0/2] powerpc/kvm: Enable running guests on RT Linux References: <1424251955-308-1-git-send-email-bogdan.purcareata@freescale.com> <54E73A6C.9080500@suse.de> <54E740E7.5090806@redhat.com> <54E74A8C.30802@linutronix.de> <1424734051.4698.17.camel@freescale.com> <54EF196E.4090805@redhat.com> In-Reply-To: <54EF196E.4090805@redhat.com> Content-Type: text/plain; charset=windows-1252 Cc: linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Graf , Bogdan Purcareata , mihai.caraman@freescale.com, Thomas Gleixner , linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 02/26/2015 02:02 PM, Paolo Bonzini wrote: > > > On 24/02/2015 00:27, Scott Wood wrote: >> This isn't a host PIC driver. It's guest PIC emulation, some of which >> is indeed not suitable for a rawlock (in particular, openpic_update_irq >> which loops on the number of vcpus, with a loop body that calls >> IRQ_check() which loops over all pending IRQs). > > The question is what behavior is wanted of code that isn't quite > RT-ready. What is preferred, bugs or bad latency? > > If the answer is bad latency (which can be avoided simply by not running > KVM on a RT kernel in production), patch 1 can be applied. If the can be applied *but* makes no difference if applied or not. > answer is bugs, patch 1 is not upstream material. > > I myself prefer to have bad latency; if something takes a spinlock in > atomic context, that spinlock should be raw. If it hurts (latency), > don't do it (use the affected code). The problem, that is fixed by this s/spin_lock/raw_spin_lock/, exists only in -RT. There is no change upstream. In general we fix such things in -RT first and forward the patches upstream if possible. This convert thingy would be possible. Bug fixing comes before latency no matter if RT or not. Converting every lock into a rawlock is not always the answer. Last thing I read from Scott is that he is not entirely sure if this is the right approach or not and patch #1 was not acked-by him either. So for now I wait for Scott's feedback and maybe a backtrace :) > > Paolo Sebastian