arm64: enable the Permission Overlay Extension for EL0

Expose a HWCAP and ID_AA64MMFR3_EL1_S1POE to userspace, so they can be used to
check if the CPU supports the feature.
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20240822151113.1479789-12-joey.gouly@arm.comSigned-off-by: Will Deacon <will@kernel.org>

Documentation/arch/arm64/elf_hwcaps.rst

@@ -365,6 +365,8 @@ HWCAP2_SME_SF8DP2
 HWCAP2_SME_SF8DP4
     Functionality implied by ID_AA64SMFR0_EL1.SF8DP4 == 0b1.
 
+HWCAP2_POE
+    Functionality implied by ID_AA64MMFR3_EL1.S1POE == 0b0001.
 
 4. Unused AT_HWCAP bits
 -----------------------

arch/arm64/include/asm/hwcap.h

@@ -157,6 +157,7 @@
 #define KERNEL_HWCAP_SME_SF8FMA		__khwcap2_feature(SME_SF8FMA)
 #define KERNEL_HWCAP_SME_SF8DP4		__khwcap2_feature(SME_SF8DP4)
 #define KERNEL_HWCAP_SME_SF8DP2		__khwcap2_feature(SME_SF8DP2)
+#define KERNEL_HWCAP_POE		__khwcap2_feature(POE)
 
 /*
  * This yields a mask that user programs can use to figure out what

arch/arm64/include/uapi/asm/hwcap.h

@@ -122,5 +122,6 @@
 #define HWCAP2_SME_SF8FMA	(1UL << 60)
 #define HWCAP2_SME_SF8DP4	(1UL << 61)
 #define HWCAP2_SME_SF8DP2	(1UL << 62)
+#define HWCAP2_POE		(1UL << 63)
 
 #endif /* _UAPI__ASM_HWCAP_H */

arch/arm64/kernel/cpufeature.c

@@ -466,6 +466,8 @@ static const struct arm64_ftr_bits ftr_id_aa64mmfr2[] = {
 };
 
 static const struct arm64_ftr_bits ftr_id_aa64mmfr3[] = {
+	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_POE),
+		       FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64MMFR3_EL1_S1POE_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64MMFR3_EL1_S1PIE_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64MMFR3_EL1_TCRX_SHIFT, 4, 0),
 	ARM64_FTR_END,
@@ -2348,6 +2350,14 @@ static void cpu_enable_mops(const struct arm64_cpu_capabilities *__unused)
 	sysreg_clear_set(sctlr_el1, 0, SCTLR_EL1_MSCEn);
 }
 
+#ifdef CONFIG_ARM64_POE
+static void cpu_enable_poe(const struct arm64_cpu_capabilities *__unused)
+{
+	sysreg_clear_set(REG_TCR2_EL1, 0, TCR2_EL1x_E0POE);
+	sysreg_clear_set(CPACR_EL1, 0, CPACR_ELx_E0POE);
+}
+#endif
+
 /* Internal helper functions to match cpu capability type */
 static bool
 cpucap_late_cpu_optional(const struct arm64_cpu_capabilities *cap)
@@ -2876,6 +2886,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.capability = ARM64_HAS_S1POE,
 		.type = ARM64_CPUCAP_BOOT_CPU_FEATURE,
 		.matches = has_cpuid_feature,
+		.cpu_enable = cpu_enable_poe,
 		ARM64_CPUID_FIELDS(ID_AA64MMFR3_EL1, S1POE, IMP)
 	},
 #endif
@@ -3043,6 +3054,9 @@ static const struct arm64_cpu_capabilities arm64_elf_hwcaps[] = {
 	HWCAP_CAP(ID_AA64FPFR0_EL1, F8DP2, IMP, CAP_HWCAP, KERNEL_HWCAP_F8DP2),
 	HWCAP_CAP(ID_AA64FPFR0_EL1, F8E4M3, IMP, CAP_HWCAP, KERNEL_HWCAP_F8E4M3),
 	HWCAP_CAP(ID_AA64FPFR0_EL1, F8E5M2, IMP, CAP_HWCAP, KERNEL_HWCAP_F8E5M2),
+#ifdef CONFIG_ARM64_POE
+	HWCAP_CAP(ID_AA64MMFR3_EL1, S1POE, IMP, CAP_HWCAP, KERNEL_HWCAP_POE),
+#endif
 	{},
 };
 

arch/arm64/kernel/cpuinfo.c

@@ -143,6 +143,7 @@ static const char *const hwcap_str[] = {
 	[KERNEL_HWCAP_SME_SF8FMA]	= "smesf8fma",
 	[KERNEL_HWCAP_SME_SF8DP4]	= "smesf8dp4",
 	[KERNEL_HWCAP_SME_SF8DP2]	= "smesf8dp2",
+	[KERNEL_HWCAP_POE]		= "poe",
 };
 
 #ifdef CONFIG_COMPAT