linux-kernel - Re: [PATCH v8 04/12] arm64: support WFET in smp_cond_relaxed

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Message-ID: <874iovp34a.fsf@oracle.com>
Date: Fri, 09 Jan 2026 01:05:57 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Will Deacon <will@...nel.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, bpf@...r.kernel.org, arnd@...db.de,
        catalin.marinas@....com, peterz@...radead.org,
        akpm@...ux-foundation.org, mark.rutland@....com, harisokn@...zon.com,
        cl@...two.org, ast@...nel.org, rafael@...nel.org,
        daniel.lezcano@...aro.org, memxor@...il.com, zhenglifeng1@...wei.com,
        xueshuai@...ux.alibaba.com, joao.m.martins@...cle.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v8 04/12] arm64: support WFET in smp_cond_relaxed_timeout()


Will Deacon <will@...nel.org> writes:

> On Sun, Dec 14, 2025 at 08:49:11PM -0800, Ankur Arora wrote:
>> +#define __CMPWAIT_CASE(w, sfx, sz)						\
>> +static inline void __cmpwait_case_##sz(volatile void *ptr,			\
>> +				       unsigned long val,			\
>> +				       s64 timeout_ns)				\
>> +{										\
>> +	unsigned long tmp;							\
>> +										\
>> +	if (!alternative_has_cap_unlikely(ARM64_HAS_WFXT) || timeout_ns <= 0) {	\
>> +		asm volatile(							\
>> +		"	sevl\n"							\
>> +		"	wfe\n"							\
>> +		"	ldxr" #sfx "\t%" #w "[tmp], %[v]\n"			\
>> +		"	eor	%" #w "[tmp], %" #w "[tmp], %" #w "[val]\n"	\
>> +		"	cbnz	%" #w "[tmp], 1f\n"				\
>> +		"	wfe\n"							\
>> +		"1:"								\
>> +		: [tmp] "=&r" (tmp), [v] "+Q" (*(u##sz *)ptr)			\
>> +		: [val] "r" (val));						\
>> +	} else {								\
>> +		u64 ecycles = arch_timer_read_counter() +			\
>> +				NSECS_TO_CYCLES(timeout_ns);			\
>> +		asm volatile(							\
>> +		"	sevl\n"							\
>> +		"	wfe\n"							\
>> +		"	ldxr" #sfx "\t%" #w "[tmp], %[v]\n"			\
>> +		"	eor	%" #w "[tmp], %" #w "[tmp], %" #w "[val]\n"	\
>> +		"	cbnz	%" #w "[tmp], 2f\n"				\
>> +		"	msr s0_3_c1_c0_0, %[ecycles]\n"				\
>> +		"2:"								\
>> +		: [tmp] "=&r" (tmp), [v] "+Q" (*(u##sz *)ptr)			\
>> +		: [val] "r" (val), [ecycles] "r" (ecycles));			\
>> +	}									\
>
> Why not have a separate helper for the WFXT version and avoid the runtime
> check on timeout_ns?

My main reason for keeping them together was that a separate helper
needed duplication of a lot of the __CMPWAIT_CASE and __CMPWAIT_GEN
stuff.

Relooking at it, I think we could get by without duplicating the
__CMPWAIT_GEN (the WFE helper just needs to take an unused timeout_ns
paramter).

But, it seems overkill to get rid of the unnecessary check on timeout_ns
(which AFAICT should be well predicted) and the duplicate static branch.

--
ankur