From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751850AbeDCOkt (ORCPT ); Tue, 3 Apr 2018 10:40:49 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:37773 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751247AbeDCOkr (ORCPT ); Tue, 3 Apr 2018 10:40:47 -0400 X-Google-Smtp-Source: AIpwx4/tcsjG8H4cW8CyefIC2OTvbCk+OlBc5T/i/7yBOj4Yk4jxHzQf0yV11U73ZsdPatEjpGBhAA== Date: Tue, 3 Apr 2018 16:40:43 +0200 From: Ingo Molnar To: "Ghannam, Yazen" Cc: "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "bp@suse.de" Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD systems Message-ID: <20180403144043.23o5aijs4hlyj2dj@gmail.com> References: <20180402183424.48222-1-Yazen.Ghannam@amd.com> <20180403110352.izr2kvsd3ys4drjq@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Ghannam, Yazen wrote: > > -----Original Message----- > > From: Ingo Molnar On Behalf Of Ingo Molnar > > Sent: Tuesday, April 3, 2018 7:04 AM > > To: Ghannam, Yazen > > Cc: x86@kernel.org; linux-kernel@vger.kernel.org; bp@suse.de > > Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD > > systems > > > > > > * Yazen Ghannam wrote: > > > > > From: Yazen Ghannam > > > > > > Recent AMD systems support using MWAIT for C1 state. However, MWAIT will > > > not allow deeper cstates than C1 on current systems. > > > > > > With play_dead() we expect the OS to use the deepest state available. > > > The deepest state available on AMD systems is reached through SystemIO > > > or HALT. If MWAIT is available, we use it instead of the other methods, > > > so we never reach the deepest state. > > > > > > Don't try to use MWAIT to play_dead() on AMD systems. Instead, we'll use > > > CPUIDLE to enter the deepest state advertised by firmware. If CPUIDLE is > > > not available then we fallback to HALT. > > > > > > Signed-off-by: Yazen Ghannam > > > --- > > > arch/x86/kernel/smpboot.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c > > > index ff99e2b6fc54..67cf00b25f83 100644 > > > --- a/arch/x86/kernel/smpboot.c > > > +++ b/arch/x86/kernel/smpboot.c > > > @@ -1536,6 +1536,9 @@ static inline void mwait_play_dead(void) > > > void *mwait_ptr; > > > int i; > > > > > > + /* Don't try native MWAIT on AMD. Stick to CPUIDLE and HALT. */ > > > + if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) > > > + return; > > > > The comment should mainly explain the 'why is this done', not the 'what is done' > > which is pretty obvious from the code ... > > > > Yes, I'll drop that comment since the commit message has the explanation. Or rather, explain the 'why' in the comment, because otherwise this is a pretty obscure condition that is not self-documenting? Thanks, Ingo