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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 71802C282CE for ; Tue, 4 Jun 2019 09:26:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4FBDC24619 for ; Tue, 4 Jun 2019 09:26:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727061AbfFDJ0o (ORCPT ); Tue, 4 Jun 2019 05:26:44 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:38780 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726933AbfFDJ0o (ORCPT ); Tue, 4 Jun 2019 05:26:44 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4D819A78; Tue, 4 Jun 2019 02:26:43 -0700 (PDT) Received: from e103592.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C62673F246; Tue, 4 Jun 2019 02:26:41 -0700 (PDT) Date: Tue, 4 Jun 2019 10:26:39 +0100 From: Dave Martin To: Viresh Kumar Cc: Catalin Marinas , Vincent Guittot , Marc Zyngier , Will Deacon , linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present() Message-ID: <20190604092639.GS28398@e103592.cambridge.arm.com> References: <699121e5c938c6f4b7b14a7e2648fa15af590a4a.1559623368.git.viresh.kumar@linaro.org> <20190604084349.prnnvjvjaeuhsmgs@mbp> <20190604085545.hsmxfqkpt2cbrhtw@vireshk-i7> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190604085545.hsmxfqkpt2cbrhtw@vireshk-i7> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 04, 2019 at 02:25:45PM +0530, Viresh Kumar wrote: > On 04-06-19, 09:43, Catalin Marinas wrote: > > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > > > We currently get following compilation warning: > > > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > > > Since the vq_present() function does not modify the vqs array, I don't > > understand why this warning. Compiler bug? > > Probably yes. Also marking array argument to functions as const is a > right thing to do, to declare that the function wouldn't change the > array values. > > I tried a recent toolchain and this doesn't happen anymore. > > Sorry for the noise. Sparse is already warning about this, but I had dismissed it as a false positive. I think this is an instance of disallowing implicit conversions of the form T ** -> T const ** because this allows a const pointer to be silently de-consted, e.g.: static const T bar; void foo(T const **p) { *p = &bar; } T *baz(void) { T *q; foo(&q); return q; } I _suspect_ that what's going on here is that the compiler is eliminating a level of indirection during inlining (i.e. converting pass-by-reference to direct access, which is precisely what I wanted to happen). This removes the potentially invalid behaviour as a side-effect. This relies on the compiler optimising / analysing the code aggressively enough though. So, I don't have a problem with dropping the extra extra const, e.g.: static bool vq_present( u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], unsigned int vq) Since this function is static and only used very locally, I don't see a big risk: the only reason for the extra const was to check that vq_present() doesn't modify vqs when it shouldn't. But it's a trivial function, and the intent is pretty clear without the extra type modifier. I'm in two minds about whether this is worth fixing, but if you want to post a patch to remove the extra const (or convert vq_present() to a macro), I'll take a look at it. Cheers ---Dave