[PATCH v5] arm64: run-time detection for aarch32 support

[Yury Norov]

Kernel option COMPAT defines the ability of executing aarch32 binaries.
Some platforms does not support aarch32 mode, and so cannot execute that
binaries. But we cannot just disable COMPAT for them because the same
kernel binary may be used by multiple platforms.

In this patch, system_supports_aarch32_el0() is introduced to detect
aarch32 support at run-time.

v4: use new CPU Feature API.
v5: use cpuid_feature_extract_field.

Signed-off-by: Yury Norov <ynorov at caviumnetworks.com>
---
 arch/arm64/include/asm/cpufeature.h | 7 +++++++
 arch/arm64/include/asm/elf.h        | 6 ++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 8f271b8..bcb0dbe 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -184,6 +184,13 @@ static inline bool system_supports_mixed_endian_el0(void)
 	return id_aa64mmfr0_mixed_endian_el0(read_system_reg(SYS_ID_AA64MMFR0_EL1));
 }
 
+static inline bool system_supports_aarch32_el0(void)
+{
+	u64 pfr0 = read_system_reg(SYS_ID_AA64PFR0_EL1);
+	return cpuid_feature_extract_field(pfr0, ID_AA64PFR0_EL0_SHIFT)
+						!= ID_AA64PFR0_EL0_64BIT_ONLY;
+}
+
 #endif /* __ASSEMBLY__ */
 
 #endif
diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index faad6df..99ec9ac 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -21,6 +21,7 @@
 /*
  * ELF register definitions..
  */
+#include <asm/cpufeature.h>
 #include <asm/ptrace.h>
 #include <asm/user.h>
 
@@ -173,8 +174,9 @@ typedef compat_elf_greg_t		compat_elf_gregset_t[COMPAT_ELF_NGREG];
 
 /* AArch32 EABI. */
 #define EF_ARM_EABI_MASK		0xff000000
-#define compat_elf_check_arch(x)	(((x)->e_machine == EM_ARM) && \
-					 ((x)->e_flags & EF_ARM_EABI_MASK))
+#define compat_elf_check_arch(x)	(system_supports_aarch32_el0()	\
+					 && ((x)->e_machine == EM_ARM)	\
+					 && ((x)->e_flags & EF_ARM_EABI_MASK))
 
 #define compat_start_thread		compat_start_thread
 #define COMPAT_SET_PERSONALITY(ex)	set_thread_flag(TIF_32BIT);
-- 
2.5.0