[22/37] arm64: Avoid cpus_have_const_cap() for ARM64_HAS_RNG

Commit Message

Mark Rutland Sept. 19, 2023, 9:28 a.m. UTC

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

Comments

Mark Brown Sept. 19, 2023, 11:24 a.m. UTC | #1

On Tue, Sep 19, 2023 at 10:28:35AM +0100, Mark Rutland wrote:
> 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.

Reviewed-by: Mark Brown <[e-mail]>

Patch

diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
index b0abc64f86b03..ecdb3cfcd0f8c 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)