From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH v3 01/23] arm64: alternative: Allow alternative_insn to always issue the first instruction Date: Mon, 27 Apr 2020 17:57:37 +0100 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 Return-path: Received: from foss.arm.com ([217.140.110.172]:38294 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725963AbgD0Q5l (ORCPT ); Mon, 27 Apr 2020 12:57:41 -0400 Content-Disposition: inline In-Reply-To: <20200421142603.3894-2-catalin.marinas@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: 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 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