linux-kernel - Re: [PATCH v2 2/2] riscv: introduce asm/swab.h

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Message-ID: <fd8e4380-eba3-4602-b925-d81c0afed6d8@iencinas.com>
Date: Fri, 21 Mar 2025 22:07:41 +0100
From: Ignacio Encinas Rubio <ignacio@...cinas.com>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Alexandre Ghiti <alex@...ti.fr>,
 Arnd Bergmann <arnd@...db.de>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev,
 skhan@...uxfoundation.org, Zhihang Shao <zhihang.shao.iscas@...il.com>,
 Björn Töpel <bjorn@...nel.org>,
 linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 2/2] riscv: introduce asm/swab.h



On 21/3/25 4:37, Eric Biggers wrote:
> On Wed, Mar 19, 2025 at 10:09:46PM +0100, Ignacio Encinas wrote:
>> +#define ARCH_SWAB(size) \
>> +static __always_inline unsigned long __arch_swab##size(__u##size value) \
>> +{									\
>> +	unsigned long x = value;					\
>> +									\
>> +	asm goto(ALTERNATIVE("j %l[legacy]", "nop", 0,			\
>> +			     RISCV_ISA_EXT_ZBB, 1)			\
>> +			     :::: legacy);				\
> 
> Is there a reason to use this instead of
> riscv_has_extension_likely(RISCV_ISA_EXT_ZBB) which seems to do the same thing,
> including using a static branch?

I just followed what's already in arch/riscv/include/asm/bitops.h

However, I changed it to

	if(riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)) {
		asm volatile (".option push\n"
			      ".option arch,+zbb\n"
			      "rev8 %0, %1\n"
			      ".option pop\n"
			      : "=r" (x) : "r" (x));
		return x >> (BITS_PER_LONG - size);
	}

	return  ___constant_swab##size(value);

and it seems gcc generates the exact same code. I tested it with 
arch/riscv/lib/csum.c (which uses swab32) and both versions generate the
exact same object file.

This certainly looks easier to read. If there are no complaints I'll
send a v3 using a plain if with riscv_has_extension_likely.

Thanks for pointing it out!