diff --git a/sys/alpha/alpha/mp_machdep.c b/sys/alpha/alpha/mp_machdep.c
index c1b9706d8118..e548d4f5d39b 100644
--- a/sys/alpha/alpha/mp_machdep.c
+++ b/sys/alpha/alpha/mp_machdep.c
@@ -62,6 +62,7 @@ static volatile int aps_ready = 0;
 static struct mtx ap_boot_mtx;
 
 u_int64_t boot_cpu_id;
+struct pcb stoppcbs[MAXCPU];
 
 static void	release_aps(void *dummy);
 static int	smp_cpu_enabled(struct pcs *pcsp);
@@ -543,11 +544,12 @@ smp_handle_ipi(struct trapframe *frame)
 
 		case IPI_STOP:
 			CTR0(KTR_SMP, "IPI_STOP");
-			atomic_set_int(&stopped_cpus, cpumask);
+			savectx(&stoppcbs[PCPU_GET(cpuid)]);
+			atomic_set_acq_int(&stopped_cpus, cpumask);
 			while ((started_cpus & cpumask) == 0)
-				alpha_mb();
-			atomic_clear_int(&started_cpus, cpumask);
-			atomic_clear_int(&stopped_cpus, cpumask);
+				cpu_spinwait();
+			atomic_clear_rel_int(&started_cpus, cpumask);
+			atomic_clear_rel_int(&stopped_cpus, cpumask);
 			break;
 		}
 	}
diff --git a/sys/alpha/include/smp.h b/sys/alpha/include/smp.h
index 9660d098ee6b..9bfd0c6eec90 100644
--- a/sys/alpha/include/smp.h
+++ b/sys/alpha/include/smp.h
@@ -27,6 +27,7 @@
 #ifndef LOCORE
 
 extern u_int64_t		boot_cpu_id;
+extern struct pcb		stoppcbs[];
 
 void	ipi_selected(u_int cpus, u_int64_t ipi);
 void	ipi_all(u_int64_t ipi);
diff --git a/sys/kern/subr_kdb.c b/sys/kern/subr_kdb.c
index 634343ce553e..2f5d1623d955 100644
--- a/sys/kern/subr_kdb.c
+++ b/sys/kern/subr_kdb.c
@@ -43,7 +43,7 @@ __FBSDID("$FreeBSD$");
 #include <machine/pcb.h>
 
 #ifdef SMP
-#if defined (__i386__) || defined(__amd64__)
+#if defined (__i386__) || defined(__amd64__) || defined(__sparc64__) || defined(__alpha__)
 #define	HAVE_STOPPEDPCBS
 #include <machine/smp.h>
 #endif
diff --git a/sys/sparc64/include/smp.h b/sys/sparc64/include/smp.h
index b51425d07476..09b93d38eced 100644
--- a/sys/sparc64/include/smp.h
+++ b/sys/sparc64/include/smp.h
@@ -72,6 +72,8 @@ struct ipi_tlb_args {
 
 struct pcpu;
 
+extern struct pcb stoppcbs[];
+
 void	cpu_mp_bootstrap(struct pcpu *pc);
 void	cpu_mp_shutdown(void);
 
diff --git a/sys/sparc64/sparc64/mp_machdep.c b/sys/sparc64/sparc64/mp_machdep.c
index 140f94416d38..ff2a0d4bd55d 100644
--- a/sys/sparc64/sparc64/mp_machdep.c
+++ b/sys/sparc64/sparc64/mp_machdep.c
@@ -82,6 +82,7 @@ __FBSDID("$FreeBSD$");
 #include <machine/md_var.h>
 #include <machine/metadata.h>
 #include <machine/ofw_machdep.h>
+#include <machine/pcb.h>
 #include <machine/smp.h>
 #include <machine/tick.h>
 #include <machine/tlb.h>
@@ -99,6 +100,7 @@ static ih_func_t cpu_ipi_stop;
 struct	cpu_start_args cpu_start_args = { 0, -1, -1, 0, 0 };
 struct	ipi_cache_args ipi_cache_args;
 struct	ipi_tlb_args ipi_tlb_args;
+struct	pcb stoppcbs[MAXCPU];
 
 struct	mtx ipi_mtx;
 
@@ -395,6 +397,7 @@ cpu_ipi_stop(struct trapframe *tf)
 {
 
 	CTR1(KTR_SMP, "cpu_ipi_stop: stopped %d", PCPU_GET(cpuid));
+	savectx(&stoppcbs[PCPU_GET(cpuid)]);
 	atomic_set_acq_int(&stopped_cpus, PCPU_GET(cpumask));
 	while ((started_cpus & PCPU_GET(cpumask)) == 0) {
 		if ((shutdown_cpus & PCPU_GET(cpumask)) != 0) {