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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 4E3E8C54FD0 for ; Mon, 27 Apr 2020 16:57:44 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 1887F206D4 for ; Mon, 27 Apr 2020 16:57:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1887F206D4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 59FCE8E0006; Mon, 27 Apr 2020 12:57:43 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 54FC48E0001; Mon, 27 Apr 2020 12:57:43 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 48BBD8E0006; Mon, 27 Apr 2020 12:57:43 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 2E0748E0001 for ; Mon, 27 Apr 2020 12:57:43 -0400 (EDT) Received: from smtpin25.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id BED10180AD804 for ; Mon, 27 Apr 2020 16:57:42 +0000 (UTC) X-FDA: 76754241564.25.kitty25_6067522df0326 X-HE-Tag: kitty25_6067522df0326 X-Filterd-Recvd-Size: 4015 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf39.hostedemail.com (Postfix) with ESMTP for ; Mon, 27 Apr 2020 16:57:42 +0000 (UTC) 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 0028131B; Mon, 27 Apr 2020 09:57:40 -0700 (PDT) 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 8AF013F68F; Mon, 27 Apr 2020 09:57:39 -0700 (PDT) Date: Mon, 27 Apr 2020 17:57:37 +0100 From: Dave Martin To: Catalin Marinas Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, Richard Earnshaw , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , Peter Collingbourne , linux-mm@kvack.org, Vincenzo Frascino , Will Deacon Subject: Re: [PATCH v3 01/23] arm64: alternative: Allow alternative_insn to always issue the first instruction Message-ID: <20200427165737.GD15808@arm.com> References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-2-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200421142603.3894-2-catalin.marinas@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Apr 21, 2020 at 03:25:41PM +0100, Catalin Marinas wrote: > There are situations where we do not want to disable the whole block > based on a config option, only the alternative part while keeping the > first instruction. Improve the alternative_insn assembler macro to take > a 'first_insn' argument, default 0, to preserve the current behaviour. > > Signed-off-by: Catalin Marinas > Cc: Will Deacon > --- > arch/arm64/include/asm/alternative.h | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h > index 5e5dc05d63a0..67d7cc608336 100644 > --- a/arch/arm64/include/asm/alternative.h > +++ b/arch/arm64/include/asm/alternative.h > @@ -111,7 +111,11 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > .byte \alt_len > .endm > > -.macro alternative_insn insn1, insn2, cap, enable = 1 > +/* > + * Disable the whole block if enable == 0, unless first_insn == 1 in which > + * case insn1 will always be issued but without an alternative insn2. > + */ > +.macro alternative_insn insn1, insn2, cap, enable = 1, first_insn = 0 > .if \enable > 661: \insn1 > 662: .pushsection .altinstructions, "a" > @@ -122,6 +126,8 @@ static inline void apply_alternatives_module(void *start, size_t length) { } > 664: .popsection > .org . - (664b-663b) + (662b-661b) > .org . - (662b-661b) + (664b-663b) > + .elseif \first_insn > + \insn1 This becomes quite unreadable at the invocation site, especially when invoked as "alternative_insn ..., 1". "... first_insn=1" is not much better either). I'm struggling to find non-trivial users of this that actually want the whole block to be deleted dependent on the config. Can we instead just always behave as if first_insn=1 instead? This this works intuitively as an alternative, not the current weird 3-way choice between insn1, insn2 and nothing at all. The only time that makes sense is when one of the insns is a branch that skips the block, but that's handled via the alternative_if macros instead. Behaving always like first_insn=1 provides an if-else that is statically optimised if the relevant feature is configured out, which I think is the only thing people are ever going to want. Maybe something depends on the current behaviour, but I can't see it so far... [...] Cheers ---Dave