[v3,25/38] arm64: Avoid cpus_have_const_cap() for ARM64_HAS_TLB_RANGE

Commit Message

Mark Rutland Oct. 10, 2023, 10:31 a.m. UTC

We use cpus_have_const_cap() to check for ARM64_HAS_TLB_RANGE, 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.

In the window between detecting the ARM64_HAS_TLB_RANGE cpucap and
patching alternative branches, we do not perform any TLB invalidation,
and even if we were to perform TLB invalidation here it would not be
functionally necessary to optimize this by using range invalidation.
Hence there's no need to use cpus_have_const_cap(), and
alternative_has_cap_unlikely() is sufficient.

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 <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Suzuki K Poulose <[e-mail]>
Cc: Will Deacon <[e-mail]>
---
 arch/arm64/include/asm/cpufeature.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index c581687b23a2e..bceb007f9ea3a 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -848,7 +848,7 @@  static inline bool system_supports_bti_kernel(void)
 
 static inline bool system_supports_tlb_range(void)
 {
-	return cpus_have_const_cap(ARM64_HAS_TLB_RANGE);
+	return alternative_has_cap_unlikely(ARM64_HAS_TLB_RANGE);
 }
 
 int do_emulate_mrs(struct pt_regs *regs, u32 sys_reg, u32 rt);