[v2,33/38] arm64: Avoid cpus_have_const_cap() for ARM64_WORKAROUND_1742098

Commit Message

Mark Rutland Oct. 5, 2023, 9:50 a.m. UTC

In elf_hwcap_fixup() we use cpus_have_const_cap() to check for
ARM64_WORKAROUND_1742098, but this is not necessary and cpus_have_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 ARM64_WORKAROUND_1742098 cpucap is detected and patched before
elf_hwcap_fixup() can run, and hence it is not necessary to use
cpus_have_const_cap(). We run cpus_have_const_cap() at most twice: once
after finalizing system cpucaps, and potentially once more after
detecting mismatched CPUs which support AArch32 at EL0. Due to this,
it's not necessary to optimize for many calls to elf_hwcap_fixup(), and
it's fine to use cpus_have_cap().

This patch replaces the use of cpus_have_const_cap() with
cpus_have_cap(), which will only generate the bitmap test and avoid
generating an alternative sequence, resulting in slightly simpler annd
smaller code being generated. The ARM64_WORKAROUND_1742098 cpucap is added to
cpucap_is_possible() so that code can be elided entirely when this is not
possible without requiring ifdeffery or IS_ENABLED() checks at each usage.

Signed-off-by: Mark Rutland <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: James Morse <[e-mail]>
Cc: Suzuki K Poulose <[e-mail]>
Cc: Will Deacon <[e-mail]>
---
 arch/arm64/include/asm/cpucaps.h | 2 ++
 arch/arm64/kernel/cpufeature.c   | 4 +---
 2 files changed, 3 insertions(+), 3 deletions(-)

Patch

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index 68b0a658e0828..5613d9e813b20 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -46,6 +46,8 @@  cpucap_is_possible(const unsigned int cap)
 		return IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0);
 	case ARM64_WORKAROUND_843419:
 		return IS_ENABLED(CONFIG_ARM64_ERRATUM_843419);
+	case ARM64_WORKAROUND_1742098:
+		return IS_ENABLED(CONFIG_ARM64_ERRATUM_1742098);
 	case ARM64_WORKAROUND_2658417:
 		return IS_ENABLED(CONFIG_ARM64_ERRATUM_2658417);
 	}
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9ab7e19b71762..a65c6f00deec5 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2221,10 +2221,8 @@  static void user_feature_fixup(void)
 
 static void elf_hwcap_fixup(void)
 {
-#ifdef CONFIG_ARM64_ERRATUM_1742098
-	if (cpus_have_const_cap(ARM64_WORKAROUND_1742098))
+	if (cpus_have_cap(ARM64_WORKAROUND_1742098))
 		compat_elf_hwcap2 &= ~COMPAT_HWCAP2_AES;
-#endif /* ARM64_ERRATUM_1742098 */
 }
 
 #ifdef CONFIG_KVM