diff --git a/sys/riscv/riscv/identcpu.c b/sys/riscv/riscv/identcpu.c
--- a/sys/riscv/riscv/identcpu.c
+++ b/sys/riscv/riscv/identcpu.c
@@ -142,7 +142,7 @@
 fill_elf_hwcap(void *dummy __unused)
 {
 	u_long caps[256] = {0};
-	char isa[ISA_NAME_MAXLEN];
+	char *isa;
 	u_long hwcap;
 	phandle_t node;
 	ssize_t len;
@@ -175,8 +175,7 @@
 		if (!ofw_bus_node_is_compatible(node, "riscv"))
 			continue;
 
-		len = OF_getprop(node, "riscv,isa", isa, sizeof(isa));
-		KASSERT(len <= ISA_NAME_MAXLEN, ("ISA string truncated"));
+		len = OF_getprop_alloc(node, "riscv,isa", (void **)&isa);
 		if (len == -1) {
 			if (bootverbose)
 				printf("fill_elf_hwcap: "
@@ -190,8 +189,9 @@
 		}
 
 		hwcap = 0;
-		for (i = ISA_PREFIX_LEN; i < len; i++)
+		for (i = ISA_PREFIX_LEN; i < len && isa[i] != '_'; i++)
 			hwcap |= caps[(unsigned char)isa[i]];
+		OF_prop_free(isa);
 
 		if (elf_hwcap != 0)
 			elf_hwcap &= hwcap;