Hi Suzuki,

On 25/02/2026 18:27, Yeoreum Yun wrote:

quoted

Use the CASLT instruction to swap the guest descriptor when FEAT_LSUI
is enabled, avoiding the need to clear the PAN bit.

Signed-off-by: Yeoreum Yun <redacted>
---
  arch/arm64/include/asm/cpucaps.h |  2 ++
  arch/arm64/include/asm/futex.h   | 17 +----------------
  arch/arm64/include/asm/lsui.h    | 27 +++++++++++++++++++++++++++
  arch/arm64/kvm/at.c              | 30 +++++++++++++++++++++++++++++-
  4 files changed, 59 insertions(+), 17 deletions(-)
  create mode 100644 arch/arm64/include/asm/lsui.h

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index 177c691914f8..6e3da333442e 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h

@@ -71,6 +71,8 @@ cpucap_is_possible(const unsigned int cap)
  		return true;
  	case ARM64_HAS_PMUV3:
  		return IS_ENABLED(CONFIG_HW_PERF_EVENTS);
+	case ARM64_HAS_LSUI:
+		return IS_ENABLED(CONFIG_ARM64_LSUI);
  	}
  	return true;

diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index b579e9d0964d..6779c4ad927f 100644
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h

@@ -7,11 +7,9 @@
  #include <linux/futex.h>
  #include <linux/uaccess.h>
-#include <linux/stringify.h>
-#include <asm/alternative.h>
-#include <asm/alternative-macros.h>
  #include <asm/errno.h>
+#include <asm/lsui.h>
  #define FUTEX_MAX_LOOPS	128 /* What's the largest number you can think of? */

@@ -91,8 +89,6 @@ __llsc_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
  #ifdef CONFIG_ARM64_LSUI
-#define __LSUI_PREAMBLE	".arch_extension lsui\n"
-
  #define LSUI_FUTEX_ATOMIC_OP(op, asm_op)				\
  static __always_inline int						\
  __lsui_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\

@@ -235,17 +231,6 @@ __lsui_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
  {
  	return __lsui_cmpxchg32(uaddr, oldval, newval, oval);
  }
-
-#define __lsui_llsc_body(op, ...)					\
-({									\
-	alternative_has_cap_unlikely(ARM64_HAS_LSUI) ?			\
-		__lsui_##op(__VA_ARGS__) : __llsc_##op(__VA_ARGS__);	\
-})
-
-#else	/* CONFIG_ARM64_LSUI */
-
-#define __lsui_llsc_body(op, ...)	__llsc_##op(__VA_ARGS__)
-
  #endif	/* CONFIG_ARM64_LSUI */

diff --git a/arch/arm64/include/asm/lsui.h b/arch/arm64/include/asm/lsui.h
new file mode 100644
index 000000000000..8f0d81953eb6
--- /dev/null
+++ b/arch/arm64/include/asm/lsui.h

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_LSUI_H
+#define __ASM_LSUI_H
+
+#include <linux/compiler_types.h>
+#include <linux/stringify.h>
+#include <asm/alternative.h>
+#include <asm/alternative-macros.h>
+#include <asm/cpucaps.h>
+
+#define __LSUI_PREAMBLE	".arch_extension lsui\n"
+
+#ifdef CONFIG_ARM64_LSUI
+
+#define __lsui_llsc_body(op, ...)					\
+({									\
+	alternative_has_cap_unlikely(ARM64_HAS_LSUI) ?			\
+		__lsui_##op(__VA_ARGS__) : __llsc_##op(__VA_ARGS__);	\
+})
+
+#else	/* CONFIG_ARM64_LSUI */
+
+#define __lsui_llsc_body(op, ...)	__llsc_##op(__VA_ARGS__)
+
+#endif	/* CONFIG_ARM64_LSUI */
+
+#endif	/* __ASM_LSUI_H */

diff --git a/arch/arm64/kvm/at.c b/arch/arm64/kvm/at.c
index 885bd5bb2f41..fd3c5749e853 100644
--- a/arch/arm64/kvm/at.c
+++ b/arch/arm64/kvm/at.c

@@ -9,6 +9,7 @@
  #include <asm/esr.h>
  #include <asm/kvm_hyp.h>
  #include <asm/kvm_mmu.h>
+#include <asm/lsui.h>
  static void fail_s1_walk(struct s1_walk_result *wr, u8 fst, bool s1ptw)
  {

@@ -1704,6 +1705,31 @@ int __kvm_find_s1_desc_level(struct kvm_vcpu *vcpu, u64 va, u64 ipa, int *level)
  	}
  }
+static int __lsui_swap_desc(u64 __user *ptep, u64 old, u64 new)
+{
+	u64 tmp = old;
+	int ret = 0;
+
+	uaccess_ttbr0_enable();
+
+	asm volatile(__LSUI_PREAMBLE
+		     "1: caslt	%[old], %[new], %[addr]\n"
+		     "2:\n"
+		     _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %w[ret])
+		     : [old] "+r" (old), [addr] "+Q" (*ptep), [ret] "+r" (ret)
+		     : [new] "r" (new)
+		     : "memory");
+
+	uaccess_ttbr0_disable();
+
+	if (ret)
+		return ret;
+	if (tmp != old)
+		return -EAGAIN;
+
+	return ret;
+}
+
  static int __lse_swap_desc(u64 __user *ptep, u64 old, u64 new)
  {
  	u64 tmp = old;

@@ -1779,7 +1805,9 @@ int __kvm_at_swap_desc(struct kvm *kvm, gpa_t ipa, u64 old, u64 new)
  		return -EPERM;
  	ptep = (u64 __user *)hva + offset;
-	if (cpus_have_final_cap(ARM64_HAS_LSE_ATOMICS))
+	if (cpucap_is_possible(ARM64_HAS_LSUI) && cpus_have_final_cap(ARM64_HAS_LSUI))

minor nit:

You don't need the cpucap_is_possible() as it is already checked via
cpus_have_final_cap()->alternative_has_cap_unlikely()

Right. It seems a little bit of redundant.
I'll remove it.

Thanks!

--
Sincerely,
Yeoreum Yun