Index: sys/arch/sparc64/include/cpu.h
===================================================================
RCS file: /cvsroot/src/sys/arch/sparc64/include/cpu.h,v
retrieving revision 1.130
diff -u -r1.130 cpu.h
--- sys/arch/sparc64/include/cpu.h	10 Mar 2020 03:49:56 -0000	1.130
+++ sys/arch/sparc64/include/cpu.h	2 Apr 2021 18:02:44 -0000
@@ -212,12 +212,14 @@
 	paddr_t			ci_devmq;  /* device mondo queue address */
 	paddr_t			ci_cpuset; /* mondo recipient address */ 
 	paddr_t			ci_mondo;  /* mondo message address */
-	
+
 	/* probe fault in PCI config space reads */
 	bool			ci_pci_probe;
 	bool			ci_pci_fault;
 
 	volatile void		*ci_ddb_regs;	/* DDB regs */
+
+	void (*ci_idlespin)(void);
 };
 
 #endif /* _KERNEL || _KMEMUSER */
@@ -438,6 +440,9 @@
 void	next_tick(long);
 void	next_stick(long);
 void	next_stick_init(void);
+#ifdef SUN4V
+void    cpu_idle_sun4v(void);
+#endif
 /* trap.c */
 void	cpu_vmspace_exec(struct lwp *, vaddr_t, vaddr_t);
 int	rwindow_save(struct lwp *);
Index: sys/arch/sparc64/sparc64/cpu.c
===================================================================
RCS file: /cvsroot/src/sys/arch/sparc64/sparc64/cpu.c,v
retrieving revision 1.138
diff -u -r1.138 cpu.c
--- sys/arch/sparc64/sparc64/cpu.c	7 Aug 2020 14:20:08 -0000	1.138
+++ sys/arch/sparc64/sparc64/cpu.c	2 Apr 2021 18:02:44 -0000
@@ -695,8 +695,22 @@
 		ci->ci_cpuset = pa;
 		pa += 64;
 	}
-	
+
+	/*
+	 * cpu_idle setup (currently only necessary for sun4v)
+	 */
+	if (CPU_ISSUN4V) {
+	  ci->ci_idlespin = cpu_idle_sun4v;
+	  printf("ci->ci_idlespin = %p\n", (void*)ci->ci_idlespin);
+	}
+}
+
+#ifdef SUN4V
+void cpu_idle_sun4v(void)
+{
+  hv_cpu_yield();
 }
+#endif
 
 int
 cpu_myid(void)
Index: sys/arch/sparc64/sparc64/genassym.cf
===================================================================
RCS file: /cvsroot/src/sys/arch/sparc64/sparc64/genassym.cf,v
retrieving revision 1.84
diff -u -r1.84 genassym.cf
--- sys/arch/sparc64/sparc64/genassym.cf	17 Feb 2020 09:09:49 -0000	1.84
+++ sys/arch/sparc64/sparc64/genassym.cf	2 Apr 2021 18:02:44 -0000
@@ -183,6 +183,7 @@
 ifdef MULTIPROCESSOR
 define	CI_IPIEVC	offsetof(struct cpu_info, ci_ipi_evcnt[0].ev_count)
 endif
+define	CI_IDLESPIN offsetof(struct cpu_info, ci_idlespin)
 
 # CPU boot arguments structure
 define	CBA_NODE	offsetof(struct cpu_bootargs, cb_node)
Index: sys/arch/sparc64/sparc64/locore.s
===================================================================
RCS file: /cvsroot/src/sys/arch/sparc64/sparc64/locore.s,v
retrieving revision 1.426
diff -u -r1.426 locore.s
--- sys/arch/sparc64/sparc64/locore.s	30 Mar 2021 20:03:14 -0000	1.426
+++ sys/arch/sparc64/sparc64/locore.s	2 Apr 2021 18:02:44 -0000
@@ -6619,11 +6619,19 @@
 	 mov %fp, %o0
 
 /*
- * nothing MD to do in the idle loop
+ * Call optional cpu_idle handler if provided
  */
 ENTRY(cpu_idle)
-	retl
+	set	CPUINFO_VA, %o0
+	LDPTR	[%o0 + CI_IDLESPIN], %o1
+	tst	%o1
+	bz	1f
+	 nop
+	jmp	%o1
 	 nop
+1:
+	retl
+	nop
 
 /*
  * cpu_switchto() switches to an lwp to run and runs it, saving the