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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 283A0C83000 for ; Tue, 28 Apr 2020 11:44:01 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id E25DC206F0 for ; Tue, 28 Apr 2020 11:44:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E25DC206F0 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 7F8308E0005; Tue, 28 Apr 2020 07:44:00 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 7A8B08E0001; Tue, 28 Apr 2020 07:44:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6BE9F8E0005; Tue, 28 Apr 2020 07:44:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0080.hostedemail.com [216.40.44.80]) by kanga.kvack.org (Postfix) with ESMTP id 5292D8E0001 for ; Tue, 28 Apr 2020 07:44:00 -0400 (EDT) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 02BB040C7 for ; Tue, 28 Apr 2020 11:44:00 +0000 (UTC) X-FDA: 76757079840.18.size33_22db895abcf12 X-HE-Tag: size33_22db895abcf12 X-Filterd-Recvd-Size: 5092 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf28.hostedemail.com (Postfix) with ESMTP for ; Tue, 28 Apr 2020 11:43:59 +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 7BE9430E; Tue, 28 Apr 2020 04:43:58 -0700 (PDT) Received: from gaia (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EE3BB3F73D; Tue, 28 Apr 2020 04:43:56 -0700 (PDT) Date: Tue, 28 Apr 2020 12:43:54 +0100 From: Catalin Marinas To: Dave Martin 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: <20200428114354.GE3868@gaia> References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-2-catalin.marinas@arm.com> <20200427165737.GD15808@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200427165737.GD15808@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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: Hi Dave, On Mon, Apr 27, 2020 at 05:57:37PM +0100, Dave P Martin wrote: > 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). That I agree. The reason I didn't leave the alternative in place here is that if gas doesn't support MTE, it will fail to compile. I wanted to avoid the several #ifdef's. > I'm struggling to find non-trivial users of this that actually want the > whole block to be deleted dependent on the config. Some of the errata stuff like CONFIG_ARM64_REPEAT_TLBI ends up with unnecessary nops. Similarly for CONFIG_ARM64_UAO/PAN and maybe a few others (it's all additional nops). We also have a few errata workaround where we didn't bother with the config enable option at all. While this is C code + inline asm, I'd like to have a consistent behaviour of ALTERNATIVE between C and .S files. Now, given that some of them (like UAO/PAN) are on by default, it probably doesn't make any difference if we always keep the first block (non-alternative). We could add a new macro ALTERNATIVE_OR_NOP. > 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... I'll give it a go in v4 and see how it looks. Another option would be an alternative_else which takes an enable argument. Thanks. -- Catalin