x86 topo_probe: do not probe smp topology if only one cpu is visible

This could lead to a division by zero if hardware is multi-core and/or
multi-threaded, but for some (quite unusual) reason FreeBSD sees only
one logical processor.  This could happen, for example, if neither MADT
nor MP Table are presented by BIOS.

Also:
- assert in topo_probe_0x4 that BSP is accounted for
- neither cpu_cores nor cpu_logical should be zero after successful
  probing, so either being zero is an indication of failed probing

Reported by:	vwe, Dan Allen <danallen46@airwired.net>
Tested by:	Dan Allen <danallen46@airwired.net>
MFC after:	3 days

1 files changed, 8 insertions, 5 deletions

diff --git a/sys/amd64/amd64/mp_machdep.c b/sys/amd64/amd64/mp_machdep.c
index 3868428..f2951da 100644
--- a/sys/amd64/amd64/mp_machdep.c
+++ b/sys/amd64/amd64/mp_machdep.c
@@ -239,6 +239,9 @@ topo_probe_0x4(void)
 			cpu_logical++;
 	}
 
+	KASSERT(cpu_cores >= 1 && cpu_logical >= 1,
+	    ("topo_probe_0x4 couldn't find BSP"));
+
 	cpu_cores /= cpu_logical;
 	hyperthreading_cpus = cpu_logical;
 }
@@ -310,7 +313,9 @@ topo_probe(void)
 		return;
 
 	logical_cpus_mask = 0;
-	if (cpu_vendor_id == CPU_VENDOR_AMD)
+	if (mp_ncpus <= 1)
+		cpu_cores = cpu_logical = 1;
+	else if (cpu_vendor_id == CPU_VENDOR_AMD)
 		topo_probe_amd();
 	else if (cpu_vendor_id == CPU_VENDOR_INTEL) {
 		/*
@@ -332,10 +337,8 @@ topo_probe(void)
 	 * Fallback: assume each logical CPU is in separate
 	 * physical package.  That is, no multi-core, no SMT.
 	 */
-	if (cpu_cores == 0)
-		cpu_cores = 1;
-	if (cpu_logical == 0)
-		cpu_logical = 1;
+	if (cpu_cores == 0 || cpu_logical == 0)
+		cpu_cores = cpu_logical = 1;
 	cpu_topo_probed = 1;
 }