Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753161AbbBWVWM (ORCPT ); Mon, 23 Feb 2015 16:22:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51370 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751591AbbBWVWL (ORCPT ); Mon, 23 Feb 2015 16:22:11 -0500 Message-ID: <54EB99E8.2060500@redhat.com> Date: Mon, 23 Feb 2015 16:21:44 -0500 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: "Maciej W. Rozycki" , Andy Lutomirski CC: Borislav Petkov , Ingo Molnar , Oleg Nesterov , X86 ML , "linux-kernel@vger.kernel.org" , Linus Torvalds Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs References: <20150221093150.GA27841@gmail.com> <20150221163840.GA32073@pd.tnic> <20150221172914.GB32073@pd.tnic> In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1839 Lines: 46 -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 02/23/2015 04:17 PM, Maciej W. Rozycki wrote: > On Sat, 21 Feb 2015, Andy Lutomirski wrote: > >>> Additionally I believe long-executing FPU instructions (i.e. >>> transcendentals) can take advantage of continuing to execute in >>> parallel where the context has already been switched rather >>> than stalling an eager FPU context switch until the FPU >>> instruction has completed. >> >> It seems highly unlikely to me that a slow FPU instruction can >> retire *after* a subsequent fxsave, which would need to happen >> for this to work. > > I meant something else -- a slow FPU instruction can retire after a > task has been switched where the FP context has been left intact, > i.e. in the lazy FP context switching case, where only the MMU > context and GPRs have been replaced. I don't think that's true, because changing the MMU context and GPRs also includes changing the instruction pointer, and changing over the execution to the new task. After a context switch, the instructions from the old task are no longer in the pipeline. - -- All rights reversed -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJU65noAAoJEM553pKExN6DyVQH+gJDWuHyoFtgKFyELFi+tTGb vB/9uR267N4Hxu0D3kedhX9HNJwBtcs1rXm++p+r4RXpz2+gl/HxihoL8v1qk4zy 1lAa5se8T5hUL7MCsD5nOIs2ca3HAGwOFtVXrjvJQ6dSCCVD/SuqJC49ENc5Nd3M 1FcW4jg+MlQ2TNJoi4QRBllmxUFffACzv8M3VWVigIibMGqrOXXSraHp0k1hFFOE LeL122JKVp7YhVj5Dx7qx1DPUkT24RT9ip1f9r5DkxFmHwdCcY31WzT4asArqHb4 uwK8LVcpTax30VHghqy8KUBVKO5h7pHvdhTYSaoLWdUJfNFJ+8Hw5sDMfPdjN2k= =6lA5 -----END PGP SIGNATURE----- -- 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/