Re: [PATCH v3 01/23] arm64: alternative: Allow alternative_insn to always issue the first instruction
From: Dave Martin <dave.martin@arm.com>
Date: 2020-04-27 16:57:44
Also in:
linux-arch, linux-mm
On Tue, Apr 21, 2020 at 03:25:41PM +0100, Catalin Marinas wrote:
quoted hunk ↗ jump to hunk
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 <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> --- 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel