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[PATCH v2 2/3] arm64: Add hypervisor safe helper for checking constant capabilities

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The hypervisor may not have full access to the kernel data structures
and hence cannot safely use cpus_have_cap() helper for checking the
system capability. Add a safe helper for hypervisors to check a constant
system capability, which *doesn't* fall back to checking the bitmap
maintained by the kernel.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
 arch/arm64/include/asm/cpufeature.h | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 758d74f..ae5e994 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -9,8 +9,6 @@
 #ifndef __ASM_CPUFEATURE_H
 #define __ASM_CPUFEATURE_H
 
-#include <linux/jump_label.h>
-
 #include <asm/hwcap.h>
 #include <asm/sysreg.h>
 
@@ -45,6 +43,8 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/bug.h>
+#include <linux/jump_label.h>
 #include <linux/kernel.h>
 
 /* CPU feature register tracking */
@@ -122,6 +122,16 @@ static inline bool cpu_have_feature(unsigned int num)
 	return elf_hwcap & (1UL << num);
 }
 
+/* System capability check for constant caps */
+static inline bool cpus_have_const_cap(int num)
+{
+	if (__builtin_constant_p(num))
+		return static_branch_unlikely(&cpu_hwcap_keys[num]);
+	BUILD_BUG();
+	/* unreachable */
+	return false;
+}
+
 static inline bool cpus_have_cap(unsigned int num)
 {
 	if (num >= ARM64_NCAPS)
@@ -218,7 +228,7 @@ static inline bool cpu_supports_mixed_endian_el0(void)
 
 static inline bool system_supports_32bit_el0(void)
 {
-	return cpus_have_cap(ARM64_HAS_32BIT_EL0);
+	return cpus_have_const_cap(ARM64_HAS_32BIT_EL0);
 }
 
 static inline bool system_supports_mixed_endian_el0(void)
-- 
2.7.4

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[PATCH v2 2/3] arm64: Add hypervisor safe helper for checking constant capabilities Suzuki K Poulose <suzuki.poulose@arm.com> - 2016-10-31 17:10 +0100
  Re: [PATCH v2 2/3] arm64: Add hypervisor safe helper for checking  constant capabilities Will Deacon <will.deacon@arm.com> - 2016-11-01 15:10 +0100
    Re: [PATCH v2 2/3] arm64: Add hypervisor safe helper for checking  constant capabilities Suzuki K Poulose <Suzuki.Poulose@arm.com> - 2016-11-01 15:40 +0100

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