From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [PATCH v3 01/23] arm64: alternative: Allow alternative_insn to always issue the first instruction Date: Tue, 28 Apr 2020 12:43:54 +0100 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 Return-path: Received: from foss.arm.com ([217.140.110.172]:49966 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726426AbgD1Ln7 (ORCPT ); Tue, 28 Apr 2020 07:43:59 -0400 Content-Disposition: inline In-Reply-To: <20200427165737.GD15808@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: 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 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