Since Armv9.6, FEAT_LSUI introduces load/store instructions that allow
privileged code to access user memory without clearing the PSTATE.PAN bit.

Add CPU feature detection for FEAT_LSUI and enable its use
when FEAT_PAN is present so that removes the need for SW_PAN handling
when using LSUI instructions.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
---
 arch/arm64/kernel/cpufeature.c | 27 +++++++++++++++++++++++++++
 arch/arm64/tools/cpucaps       |  1 +
 2 files changed, 28 insertions(+)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c840a93b9ef9..b41ea479c868 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -280,6 +280,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar2[] = {
 
 static const struct arm64_ftr_bits ftr_id_aa64isar3[] = {
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR3_EL1_FPRCVT_SHIFT, 4, 0),
+	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR3_EL1_LSUI_SHIFT, 4, ID_AA64ISAR3_EL1_LSUI_NI),
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR3_EL1_LSFE_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64ISAR3_EL1_FAMINMAX_SHIFT, 4, 0),
 	ARM64_FTR_END,
@@ -2509,6 +2510,23 @@ test_has_gicv5_legacy(const struct arm64_cpu_capabilities *entry, int scope)
 	return !!(read_sysreg_s(SYS_ICC_IDR0_EL1) & ICC_IDR0_EL1_GCIE_LEGACY);
 }
 
+#ifdef CONFIG_ARM64_LSUI
+static bool has_lsui(const struct arm64_cpu_capabilities *entry, int scope)
+{
+	if (!has_cpuid_feature(entry, scope))
+		return false;
+
+	/*
+	 * A CPU that supports LSUI should also support FEAT_PAN,
+	 * so that SW_PAN handling is not required.
+	 */
+	if (WARN_ON(!__system_matches_cap(ARM64_HAS_PAN)))
+		return false;
+
+	return true;
+}
+#endif
+
 static const struct arm64_cpu_capabilities arm64_features[] = {
 	{
 		.capability = ARM64_ALWAYS_BOOT,
@@ -3148,6 +3166,15 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.matches = has_cpuid_feature,
 		ARM64_CPUID_FIELDS(ID_AA64MMFR1_EL1, XNX, IMP)
 	},
+#ifdef CONFIG_ARM64_LSUI
+	{
+		.desc = "Unprivileged Load Store Instructions (LSUI)",
+		.capability = ARM64_HAS_LSUI,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.matches = has_lsui,
+		ARM64_CPUID_FIELDS(ID_AA64ISAR3_EL1, LSUI, IMP)
+	},
+#endif
 	{},
 };
 
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 0fac75f01534..4b2f7f3f2b80 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -46,6 +46,7 @@ HAS_HCX
 HAS_LDAPR
 HAS_LPA2
 HAS_LSE_ATOMICS
+HAS_LSUI
 HAS_MOPS
 HAS_NESTED_VIRT
 HAS_BBML2_NOABORT
-- 
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}

On Wed, Jan 21, 2026 at 07:06:17PM +0000, Yeoreum Yun wrote:
> +#ifdef CONFIG_ARM64_LSUI
> +static bool has_lsui(const struct arm64_cpu_capabilities *entry, int scope)
> +{
> +	if (!has_cpuid_feature(entry, scope))
> +		return false;
> +
> +	/*
> +	 * A CPU that supports LSUI should also support FEAT_PAN,
> +	 * so that SW_PAN handling is not required.
> +	 */
> +	if (WARN_ON(!__system_matches_cap(ARM64_HAS_PAN)))
> +		return false;
> +
> +	return true;
> +}
> +#endif

I still find this artificial dependency a bit strange. Maybe one doesn't
want any PAN at all (software or hardware) and won't get LSUI either
(it's unlikely but possible).

We have the uaccess_ttbr0_*() calls already for !LSUI, so maybe
structuring the macros in a way that they also take effect with LSUI.
For futex, we could add some new functions like uaccess_enable_futex()
which wouldn't do anything if LSUI is enabled with hw PAN.

-- 
Catalin