From 1963d9660d99bdd1bd3666afc6c670d84bff0efb Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Mon, 16 Oct 2023 11:24:46 +0100 Subject: [PATCH] arm64: Avoid cpus_have_const_cap() for ARM64_HAS_RNG In __cpu_has_rng() we use cpus_have_const_cap() to check for ARM64_HAS_RNG, 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_RNG cpucap and patching alternative branches, nothing which calls __cpu_has_rng() can run, and hence it's not necessary to use cpus_have_const_cap(). 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 Reviewed-by: Mark Brown Cc: Ard Biesheuvel Cc: Suzuki K Poulose Cc: Will Deacon Signed-off-by: Catalin Marinas --- arch/arm64/include/asm/archrandom.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h index b0abc64f86b0..ecdb3cfcd0f8 100644 --- a/arch/arm64/include/asm/archrandom.h +++ b/arch/arm64/include/asm/archrandom.h @@ -63,7 +63,7 @@ static __always_inline bool __cpu_has_rng(void) { if (unlikely(!system_capabilities_finalized() && !preemptible())) return this_cpu_has_cap(ARM64_HAS_RNG); - return cpus_have_const_cap(ARM64_HAS_RNG); + return alternative_has_cap_unlikely(ARM64_HAS_RNG); } static inline size_t __must_check arch_get_random_longs(unsigned long *v, size_t max_longs) -- 2.20.1