[16/37] arm64: Avoid cpus_have_const_cap() for ARM64_HAS_CACHE_DIC

Commit Message

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

In icache_inval_all_pou() we use cpus_have_const_cap() to check for
ARM64_HAS_CACHE_DIC, 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 icache_inval_all_pou() is an
optimization to skip a redundant (but benign) IC IALLUIS + DSB ISH
sequence when all CPUs in the system have DIC. In the window between
detecting the ARM64_HAS_CACHE_DIC cpucap and patching alternative
branches there is only a single potential call to icache_inval_all_pou()
(in the alternatives patching itself), which there's no need to optimize
for at the expense of other callers.

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. This also aligns better with the way we patch the assembly
cache maintenance sequences in arch/arm64/mm/cache.S.

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/cacheflush.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/arch/arm64/include/asm/cacheflush.h b/arch/arm64/include/asm/cacheflush.h
index d115451ed263d..fefac75fa009f 100644
--- a/arch/arm64/include/asm/cacheflush.h
+++ b/arch/arm64/include/asm/cacheflush.h
@@ -132,7 +132,7 @@  void flush_dcache_folio(struct folio *);
 
 static __always_inline void icache_inval_all_pou(void)
 {
-	if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC))
+	if (alternative_has_cap_unlikely(ARM64_HAS_CACHE_DIC))
 		return;
 
 	asm("ic	ialluis");