diff --git a/sys/amd64/amd64/machdep.c b/sys/amd64/amd64/machdep.c
index fab311142445..e9d160ff2431 100644
--- a/sys/amd64/amd64/machdep.c
+++ b/sys/amd64/amd64/machdep.c
@@ -1401,10 +1401,13 @@ getmemsize(caddr_t kmdp, u_int64_t first)
 		Maxmem = atop(physmem_tunable);
 
 	/*
-	 * By default keep the memtest enabled.  Use a general name so that
+	 * By default enable the memory test on real hardware, and disable
+	 * it if we appear to be running in a VM.  This avoids touching all
+	 * pages unnecessarily, which doesn't matter on real hardware but is
+	 * bad for shared VM hosts.  Use a general name so that
 	 * one could eventually do more with the code than just disable it.
 	 */
-	memtest = 1;
+	memtest = (vm_guest > VM_GUEST_NO) ? 0 : 1;
 	TUNABLE_ULONG_FETCH("hw.memtest.tests", &memtest);
 
 	/*
diff --git a/sys/i386/i386/machdep.c b/sys/i386/i386/machdep.c
index c7352d9b8bec..e481e22f79ca 100644
--- a/sys/i386/i386/machdep.c
+++ b/sys/i386/i386/machdep.c
@@ -2369,10 +2369,13 @@ physmap_done:
 		Maxmem = atop(physmap[physmap_idx + 1]);
 
 	/*
-	 * By default keep the memtest enabled.  Use a general name so that
+	 * By default enable the memory test on real hardware, and disable
+	 * it if we appear to be running in a VM.  This avoids touching all
+	 * pages unnecessarily, which doesn't matter on real hardware but is
+	 * bad for shared VM hosts.  Use a general name so that
 	 * one could eventually do more with the code than just disable it.
 	 */
-	memtest = 1;
+	memtest = (vm_guest > VM_GUEST_NO) ? 0 : 1;
 	TUNABLE_ULONG_FETCH("hw.memtest.tests", &memtest);
 
 	if (atop(physmap[physmap_idx + 1]) != Maxmem &&