Index: head/sys/riscv/include/sbi.h
===================================================================
--- head/sys/riscv/include/sbi.h
+++ head/sys/riscv/include/sbi.h
@@ -55,6 +55,7 @@
 #define	SBI_ERR_INVALID_PARAM		-3
 #define	SBI_ERR_DENIED			-4
 #define	SBI_ERR_INVALID_ADDRESS		-5
+#define	SBI_ERR_ALREADY_AVAILABLE	-6
 
 /* SBI Base Extension */
 #define	SBI_EXT_ID_BASE			0x10
@@ -66,6 +67,16 @@
 #define	SBI_BASE_GET_MARCHID		5
 #define	SBI_BASE_GET_MIMPID		6
 
+/* Hart State Management (HSM) Extension */
+#define	SBI_EXT_ID_HSM			0x48534D
+#define	SBI_HSM_HART_START		0
+#define	SBI_HSM_HART_STOP		1
+#define	SBI_HSM_HART_STATUS		2
+#define	 SBI_HSM_STATUS_STARTED		0
+#define	 SBI_HSM_STATUS_STOPPED		1
+#define	 SBI_HSM_STATUS_START_PENDING	2
+#define	 SBI_HSM_STATUS_STOP_PENDING	3
+
 /* Legacy Extensions */
 #define	SBI_SET_TIMER			0
 #define	SBI_CONSOLE_PUTCHAR		1
@@ -127,6 +138,30 @@
 {
 	return (SBI_CALL1(SBI_EXT_ID_BASE, SBI_BASE_PROBE_EXTENSION, id).value);
 }
+
+/* Hart State Management extension functions. */
+
+/*
+ * Start execution on the specified hart at physical address start_addr. The
+ * register a0 will contain the hart's ID, and a1 will contain the value of
+ * priv.
+ */
+int sbi_hsm_hart_start(u_long hart, u_long start_addr, u_long priv);
+
+/*
+ * Stop execution on the current hart. Interrupts should be disabled, or this
+ * function may return.
+ */
+void sbi_hsm_hart_stop(void);
+
+/*
+ * Get the execution status of the specified hart. The status will be one of:
+ *  - SBI_HSM_STATUS_STARTED
+ *  - SBI_HSM_STATUS_STOPPED
+ *  - SBI_HSM_STATUS_START_PENDING
+ *  - SBI_HSM_STATUS_STOP_PENDING
+ */
+int sbi_hsm_hart_status(u_long hart);
 
 /* Legacy extension functions. */
 static __inline void
Index: head/sys/riscv/riscv/sbi.c
===================================================================
--- head/sys/riscv/riscv/sbi.c
+++ head/sys/riscv/riscv/sbi.c
@@ -113,6 +113,31 @@
 	printf("SBI Specification Version: %u.%u\n", major, minor);
 }
 
+int
+sbi_hsm_hart_start(u_long hart, u_long start_addr, u_long priv)
+{
+	struct sbi_ret ret;
+
+	ret = SBI_CALL3(SBI_EXT_ID_HSM, SBI_HSM_HART_START, hart, start_addr, priv);
+	return (ret.error != 0 ? (int)ret.error : 0);
+}
+
+void
+sbi_hsm_hart_stop(void)
+{
+	(void)SBI_CALL0(SBI_EXT_ID_HSM, SBI_HSM_HART_STOP);
+}
+
+int
+sbi_hsm_hart_status(u_long hart)
+{
+	struct sbi_ret ret;
+
+	ret = SBI_CALL1(SBI_EXT_ID_HSM, SBI_HSM_HART_STATUS, hart);
+
+	return (ret.error != 0 ? (int)ret.error : (int)ret.value);
+}
+
 void
 sbi_init(void)
 {