x86: refine nr_sockets calculation

The previous variant didn't work for non-contiguous socket numbers.

Reported-by: Ed Swierk <eswierk@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Ed Swierk <eswierk@xxxxxxxxxxxxxxxxxx>

--- a/xen/arch/x86/mpparse.c
+++ b/xen/arch/x86/mpparse.c
@@ -89,19 +89,14 @@ void __init set_nr_cpu_ids(unsigned int
 
 void __init set_nr_sockets(void)
 {
-    /*
-     * Count the actual cpus in the socket 0 and use it to calculate nr_sockets
-     * so that the latter will be always >= the actual socket number in the
-     * system even when APIC IDs from MP table are too sparse.
-     */
-    unsigned int cpus = bitmap_weight(phys_cpu_present_map.mask,
-                                      boot_cpu_data.x86_max_cores *
-                                      boot_cpu_data.x86_num_siblings);
-
-    if ( cpus == 0 )
-        cpus = 1;
-
-    nr_sockets = DIV_ROUND_UP(num_processors + disabled_cpus, cpus);
+	nr_sockets = last_physid(phys_cpu_present_map)
+		     / boot_cpu_data.x86_max_cores
+		     / boot_cpu_data.x86_num_siblings + 1;
+	if (disabled_cpus)
+		nr_sockets += (disabled_cpus - 1)
+			      / boot_cpu_data.x86_max_cores
+			      / boot_cpu_data.x86_num_siblings + 1;
+	printk(XENLOG_DEBUG "nr_sockets: %u\n", nr_sockets);
 }
 
 /*
--- a/xen/include/asm-x86/mpspec.h
+++ b/xen/include/asm-x86/mpspec.h
@@ -43,6 +43,19 @@ typedef struct physid_mask physid_mask_t
 #define physid_isset(physid, map)		test_bit(physid, (map).mask)
 #define physid_test_and_set(physid, map)	test_and_set_bit(physid, (map).mask)
 
+#define first_physid(map)			find_first_bit((map).mask, \
+							       MAX_APICS)
+#define next_physid(id, map)			find_next_bit((map).mask, \
+							      MAX_APICS, (id) + 1)
+#define last_physid(map) ({ \
+	const unsigned long *mask = (map).mask; \
+	unsigned int id, last = MAX_APICS; \
+	for (id = find_first_bit(mask, MAX_APICS); id < MAX_APICS; \
+	     id = find_next_bit(mask, MAX_APICS, (id) + 1)) \
+		last = id; \
+	last; \
+})
+
 #define physids_and(dst, src1, src2)		bitmap_and((dst).mask, (src1).mask, (src2).mask, MAX_APICS)
 #define physids_or(dst, src1, src2)		bitmap_or((dst).mask, (src1).mask, (src2).mask, MAX_APICS)
 #define physids_clear(map)			bitmap_zero((map).mask, MAX_APICS)