On Fri, Jan 09, 2026 at 01:05:57AM -0800, Ankur Arora wrote:

Will Deacon [off-list ref] writes:

quoted

On Sun, Dec 14, 2025 at 08:49:11PM -0800, Ankur Arora wrote:

quoted

+#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.

tbh, I was actually struggling to see what the check achieves. In fact,
why is 'timeout_ns' signed in the first place? Has BPF invented time
travel now? :p

If the requested timeout is 0 then we should return immediately (or issue
a WFET which will wake up immediately).

If the requested timeout is negative, then WFET may end up with a really
long timeout, but that should still be no worse than a plain WFE.

If the check is only there to de-multiplex __cmpwait() vs
__cmpwait_relaxed_timeout() as the caller, then I think we should just
avoid muxing them in the first place. The duplication argument doesn't
hold a lot of water when the asm block is already mostly copy-paste.

Will