x86/apic/32: Decrapify the def_bigsmp mechanism

If the system has more than 8 CPUs then XAPIC and the bigsmp APIC driver is
required. This is ensured via:

  1) Enumerating all possible CPUs up to NR_CPUS

  2) Checking at boot CPU APIC setup time whether the system has more than
     8 CPUs and has an XAPIC.

     If that's the case then it's attempted to install the bigsmp APIC
     driver and a magic variable 'def_to_bigsmp' is set to one.

  3) If that magic variable is set and CONFIG_X86_BIGSMP=n and the system
     has more than 8 CPUs smp_sanity_check() removes all CPUs >= #8 from
     the present and possible mask in the most convoluted way.

This logic is completely broken for the case where the bigsmp driver is
enabled, but not selected due to a command line option specifying the
default APIC. In that case the system boots with default APIC in logical
destination mode and fails to reduce the number of CPUs.

That aside the above which is sprinkled over 3 different places is yet
another piece of art.

It would have been too obvious to check the requirements upfront and limit
nr_cpu_ids _before_ enumerating tons of CPUs and then removing them again.

Implement exactly this. Check the bigsmp requirement when the boot APIC is
registered which happens _before_ ACPI/MPTABLE parsing and limit the number
of CPUs to 8 if it can't be used. Switch it over when the boot CPU apic is
set up if necessary.

[ dhansen: fix nr_cpu_ids off-by-one in default_setup_apic_routing() ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)

1 files changed, 17 insertions, 18 deletions

diff --git a/arch/x86/kernel/apic/probe_32.c b/arch/x86/kernel/apic/probe_32.c
index 20d7f571b80f..706a8447ac17 100644
--- a/arch/x86/kernel/apic/probe_32.c
+++ b/arch/x86/kernel/apic/probe_32.c
@@ -10,6 +10,8 @@
 #include <linux/errno.h>
 #include <linux/smp.h>
 
+#include <xen/xen.h>
+
 #include <asm/io_apic.h>
 #include <asm/apic.h>
 #include <asm/acpi.h>
@@ -123,36 +125,33 @@ static int __init parse_apic(char *arg)
 }
 early_param("apic", parse_apic);
 
-void __init default_setup_apic_routing(void)
+void __init x86_32_probe_bigsmp_early(void)
 {
-	int version = boot_cpu_apic_version;
+	if (nr_cpu_ids <= 8 || xen_pv_domain())
+		return;
 
-	if (num_possible_cpus() > 8) {
+	if (IS_ENABLED(CONFIG_X86_BIGSMP)) {
 		switch (boot_cpu_data.x86_vendor) {
 		case X86_VENDOR_INTEL:
-			if (!APIC_XAPIC(version)) {
-				def_to_bigsmp = 0;
+			if (!APIC_XAPIC(boot_cpu_apic_version))
 				break;
-			}
 			/* P4 and above */
 			fallthrough;
 		case X86_VENDOR_HYGON:
 		case X86_VENDOR_AMD:
-			def_to_bigsmp = 1;
+			if (apic_bigsmp_possible(cmdline_apic))
+				return;
+			break;
 		}
 	}
+	pr_info("Limiting to 8 possible CPUs\n");
+	set_nr_cpu_ids(8);
+}
 
-#ifdef CONFIG_X86_BIGSMP
-	/*
-	 * This is used to switch to bigsmp mode when
-	 * - There is no apic= option specified by the user
-	 * - generic_apic_probe() has chosen apic_default as the sub_arch
-	 * - we find more than 8 CPUs in acpi LAPIC listing with xAPIC support
-	 */
-
-	if (!cmdline_apic && apic == &apic_default)
-		generic_bigsmp_probe();
-#endif
+void __init default_setup_apic_routing(void)
+{
+	if (nr_cpu_ids > 8 && !xen_pv_domain())
+		apic_bigsmp_force();
 
 	if (apic->setup_apic_routing)
 		apic->setup_apic_routing();