From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CCB5AC4741F for ; Thu, 5 Nov 2020 17:56:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8A1482074B for ; Thu, 5 Nov 2020 17:56:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726996AbgKER4y (ORCPT ); Thu, 5 Nov 2020 12:56:54 -0500 Received: from foss.arm.com ([217.140.110.172]:38944 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726214AbgKER4y (ORCPT ); Thu, 5 Nov 2020 12:56:54 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2F7DC14BF; Thu, 5 Nov 2020 09:56:53 -0800 (PST) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B88293F719; Thu, 5 Nov 2020 09:56:51 -0800 (PST) Date: Thu, 5 Nov 2020 17:56:48 +0000 From: Dave Martin To: Mark Brown Cc: Alexandre Torgue , Catalin Marinas , Ard Biesheuvel , l00374334 , Linux Crypto Mailing List , Maxime Coquelin , Will Deacon , "David S. Miller" , Linux ARM , Herbert Xu Subject: Re: [PATCH 1/1] arm64: Accelerate Adler32 using arm64 SVE instructions. Message-ID: <20201105175647.GI6882@arm.com> References: <20201103121506.1533-1-liqiang64@huawei.com> <20201103121506.1533-2-liqiang64@huawei.com> <20201103180031.GO6882@arm.com> <20201104175032.GA15020@sirena.org.uk> <20201104181256.GG6882@arm.com> <20201104184905.GB4812@sirena.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201104184905.GB4812@sirena.org.uk> User-Agent: Mutt/1.5.23 (2014-03-12) Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org On Wed, Nov 04, 2020 at 06:49:05PM +0000, Mark Brown wrote: > On Wed, Nov 04, 2020 at 06:13:06PM +0000, Dave Martin wrote: > > On Wed, Nov 04, 2020 at 05:50:33PM +0000, Mark Brown wrote: > > > > I think at a minimum we'd want to handle the vector length explicitly > > > for kernel mode SVE, vector length independent code will work most of > > > the time but at the very least it feels like a landmine waiting to cause > > > trouble. If nothing else there's probably going to be cases where it > > > makes a difference for performance. Other than that I'm not currently > > ... > > > The main reasons for constraining the vector length are a) to hide > > mismatches between CPUs in heterogeneous systems, b) to ensure that > > validated software doesn't run with a vector length it wasn't validated > > for, and c) testing. > > > For kernel code, it's reasonable to say that all code should be vector- > > length agnostic unless there's a really good reason not to be. So we > > may not care too much about (b). > > > In that case, just setting ZCR_EL1.LEN to max in kernel_sve_begin() (or > > whatever) probably makes sense. > > I agree, that's most likely a good default. > > > For (c), it might be useful to have a command-line parameter or debugfs > > widget to constrain the vector length for kernel code; perhaps globally > > or perhaps per driver or algo. > > I think a global control would be good for testing, it seems simpler and > easier all round. The per thing tuning seems more useful for cases > where we run into something like a performance reason to use a limited > set of vector lengths but I think we should only add that when we have > at least one user for it, some examples of actual restrictions we want > would probably be helpful for designing the interface. Ack; note that an algo that wants to use a particular vector length can do so by means of the special predicate patterns VLnnn, POW2, MUL3 etc. So setting an explicit limit in ZCR_EL1.LEN should hopefully be an uncommon requirement. > > > Nonetheless, working up a candidate algorithm to help us see whether > > there is a good use case seems like a worthwhile project, so I don't > > want to discourage that too much. > > Definitely worth exploring. Cheers ---Dave