diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
index 10bd7b170118..bff68004ad43 100644
--- a/Documentation/arch/riscv/hwprobe.rst
+++ b/Documentation/arch/riscv/hwprobe.rst
@@ -165,6 +165,10 @@ The following keys are defined:
        defined in the RISC-V ISA manual starting from commit 5618fb5a216b
        ("Ztso is now ratified.")
 
+  * :c:macro:`RISCV_HWPROBE_EXT_ZACAS`: The Zacas extension is supported as
+       defined in the Atomic Compare-and-Swap (CAS) instructions manual starting
+       from commit 5059e0ca641c ("update to ratified").
+
 * :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains performance
   information about the selected set of processors.
 
diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
index 01ac3dc196e5..ac65bb43c8e7 100644
--- a/arch/riscv/include/uapi/asm/hwprobe.h
+++ b/arch/riscv/include/uapi/asm/hwprobe.h
@@ -57,6 +57,7 @@ struct riscv_hwprobe {
 #define		RISCV_HWPROBE_EXT_ZVFHMIN	(1 << 31)
 #define		RISCV_HWPROBE_EXT_ZFA		(1ULL << 32)
 #define		RISCV_HWPROBE_EXT_ZTSO		(1ULL << 33)
+#define		RISCV_HWPROBE_EXT_ZACAS		(1ULL << 34)
 #define RISCV_HWPROBE_KEY_CPUPERF_0	5
 #define		RISCV_HWPROBE_MISALIGNED_UNKNOWN	(0 << 0)
 #define		RISCV_HWPROBE_MISALIGNED_EMULATED	(1 << 0)
diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index 6564fa9e7a7f..6c680c75ac0d 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -175,6 +175,7 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
 		EXT_KEY(ZKT);
 		EXT_KEY(ZIHINTNTL);
 		EXT_KEY(ZTSO);
+		EXT_KEY(ZACAS);
 
 		if (has_vector()) {
 			EXT_KEY(ZVBB);