[PATCH] arm64: use const cap for system_uses_ttbr0_pan()

Mark Rutland mark.rutland at arm.com
Fri Mar 10 09:44:18 PST 2017

Since commit 4b65a5db362783ab ("arm64: Introduce
uaccess_{disable,enable} functionality based on TTBR0_EL1"),
system_uses_ttbr0_pan() has used cpus_have_cap() to determine whether
PAN is present.

Since commit a4023f682739439b ("arm64: Add hypervisor safe helper for
checking constant capabilities"), which was introduced around the same
time, cpus_have_cap() doesn't try to use a static key, and must always
perform a load, test, and consitional branch (likely a tbnz for the
latter two).

Elsewhere, we moved to using cpus_have_const_cap(), which can use a
static key (i.e. a non-conditional branch), which is patched at runtime
when the feature is detected.

This patch makes system_uses_ttbr0_pan() use cpus_have_const_cap(). The
static key is likely a win for hot-paths like the uacccess primitives,
and this makes our usage consistent regardless.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
 arch/arm64/include/asm/cpufeature.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 05310ad..f31c48d 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -251,7 +251,7 @@ static inline bool system_supports_fpsimd(void)
 static inline bool system_uses_ttbr0_pan(void)
-		!cpus_have_cap(ARM64_HAS_PAN);
+		!cpus_have_const_cap(ARM64_HAS_PAN);
 #endif /* __ASSEMBLY__ */

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