[PATCH v3 24/38] arm64: Avoid cpus_have_const_cap() for ARM64_HAS_WFXT

[Mark Rutland]

In __delay() we use cpus_have_const_cap() to check for ARM64_HAS_WFXT,
but this is not necessary and alternative_has_cap() would be preferable.

For historical reasons, cpus_have_const_cap() is more complicated than
it needs to be. Before cpucaps are finalized, it will perform a bitmap
test of the system_cpucaps bitmap, and once cpucaps are finalized it
will use an alternative branch. This used to be necessary to handle some
race conditions in the window between cpucap detection and the
subsequent patching of alternatives and static branches, where different
branches could be out-of-sync with one another (or w.r.t. alternative
sequences). Now that we use alternative branches instead of static
branches, these are all patched atomically w.r.t. one another, and there
are only a handful of cases that need special care in the window between
cpucap detection and alternative patching.

Due to the above, it would be nice to remove cpus_have_const_cap(), and
migrate callers over to alternative_has_cap_*(), cpus_have_final_cap(),
or cpus_have_cap() depending on when their requirements. This will
remove redundant instructions and improve code generation, and will make
it easier to determine how each callsite will behave before, during, and
after alternative patching.

The cpus_have_const_cap() check in __delay() is an optimization to use
WFIT and WFET in preference to busy-polling the counter and/or using
regular WFE and relying upon the architected timer event stream. It is
not necessary to apply this optimization in the window between detecting
the ARM64_HAS_WFXT cpucap and patching alternatives.

This patch replaces the use of cpus_have_const_cap() with
alternative_has_cap_unlikely(), which will avoid generating code to test
the system_cpucaps bitmap and should be better for all subsequent calls
at runtime.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Marc Zyngier <maz at kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/lib/delay.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
index 5b7890139bc2f..cb2062e7e2340 100644
--- a/arch/arm64/lib/delay.c
+++ b/arch/arm64/lib/delay.c
@@ -27,7 +27,7 @@ void __delay(unsigned long cycles)
 {
 	cycles_t start = get_cycles();
 
-	if (cpus_have_const_cap(ARM64_HAS_WFXT)) {
+	if (alternative_has_cap_unlikely(ARM64_HAS_WFXT)) {
 		u64 end = start + cycles;
 
 		/*
-- 
2.30.2