x86: possibly bring up all CPUs even if not all are supposed to be used

Reportedly Intel CPUs which can't broadcast #MC to all targeted
cores/threads because some have CR4.MCE clear will shut down. Therefore
we want to keep CR4.MCE enabled when offlining a CPU, and we need to
bring up all CPUs in order to be able to set CR4.MCE in the first place.

The use of clear_in_cr4() in cpu_mcheck_disable() was ill advised
anyway, and to avoid future similar mistakes I'm removing clear_in_cr4()
altogether right here.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
master commit: 8797d20a6e
master date: 2018-07-19 13:43:33 +0100

xen/arch/x86/cpu/common.c

@@ -15,6 +15,7 @@
 #include <public/sysctl.h> /* for XEN_INVALID_{SOCKET,CORE}_ID */
 
 #include "cpu.h"
+#include "mcheck/x86_mca.h"
 
 static bool_t __cpuinitdata use_xsave = 1;
 boolean_param("xsave", use_xsave);
@@ -209,6 +210,9 @@ static void __init early_cpu_detect(void)
 	c->x86_capability[0] = cap0;
 	c->x86_capability[4] = cap4;
 
+	if (c->x86_vendor != X86_VENDOR_AMD)
+		park_offline_cpus = !mce_disabled;
+
 	initialize_cpu_data(0);
 }
 

xen/arch/x86/cpu/mcheck/mce_intel.c

@@ -628,8 +628,6 @@ static void clear_cmci(void)
 
 static void cpu_mcheck_disable(void)
 {
-    clear_in_cr4(X86_CR4_MCE);
-
     if (cmci_support && !mce_disabled)
         clear_cmci();
 }

xen/arch/x86/mpparse.c

@@ -69,19 +69,26 @@ physid_mask_t phys_cpu_present_map;
 
 void __init set_nr_cpu_ids(unsigned int max_cpus)
 {
+	unsigned int tot_cpus = num_processors + disabled_cpus;
+
 	if (!max_cpus)
-		max_cpus = num_processors + disabled_cpus;
+		max_cpus = tot_cpus;
 	if (max_cpus > NR_CPUS)
 		max_cpus = NR_CPUS;
 	else if (!max_cpus)
 		max_cpus = 1;
 	printk(XENLOG_INFO "SMP: Allowing %u CPUs (%d hotplug CPUs)\n",
 	       max_cpus, max_t(int, max_cpus - num_processors, 0));
-	nr_cpu_ids = max_cpus;
+
+	if (!park_offline_cpus)
+		tot_cpus = max_cpus;
+	nr_cpu_ids = min(tot_cpus, NR_CPUS + 0u);
+	if (park_offline_cpus && nr_cpu_ids < num_processors)
+		printk(XENLOG_WARNING "SMP: Cannot bring up %u further CPUs\n",
+		       num_processors - nr_cpu_ids);
 
 #ifndef nr_cpumask_bits
-	nr_cpumask_bits = (max_cpus + (BITS_PER_LONG - 1)) &
-			  ~(BITS_PER_LONG - 1);
+	nr_cpumask_bits = ROUNDUP(nr_cpu_ids, BITS_PER_LONG);
 	printk(XENLOG_DEBUG "NR_CPUS:%u nr_cpumask_bits:%u\n",
 	       NR_CPUS, nr_cpumask_bits);
 #endif

xen/arch/x86/setup.c

@@ -616,7 +616,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
 {
     char *memmap_type = NULL;
     char *cmdline, *kextra, *loader;
-    unsigned int initrdidx, domcr_flags = DOMCRF_s3_integrity;
+    unsigned int initrdidx, num_parked = 0, domcr_flags = DOMCRF_s3_integrity;
     multiboot_info_t *mbi = __va(mbi_p);
     module_t *mod = (module_t *)__va(mbi->mods_addr);
     unsigned long nr_pages, raw_max_page, modules_headroom, *module_map;
@@ -1319,7 +1319,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     else
     {
         set_nr_cpu_ids(max_cpus);
-        max_cpus = nr_cpu_ids;
+        if ( !max_cpus )
+            max_cpus = nr_cpu_ids;
     }
 
     /* Low mappings were only needed for some BIOS table parsing. */
@@ -1431,15 +1432,26 @@ void __init noreturn __start_xen(unsigned long mbi_p)
         /* Set up node_to_cpumask based on cpu_to_node[]. */
         numa_add_cpu(i);        
 
-        if ( (num_online_cpus() < max_cpus) && !cpu_online(i) )
+        if ( (park_offline_cpus || num_online_cpus() < max_cpus) &&
+             !cpu_online(i) )
         {
             int ret = cpu_up(i);
             if ( ret != 0 )
                 printk("Failed to bring up CPU %u (error %d)\n", i, ret);
+            else if ( num_online_cpus() > max_cpus )
+            {
+                ret = cpu_down(i);
+                if ( !ret )
+                    ++num_parked;
+                else
+                    printk("Could not re-offline CPU%u (%d)\n", i, ret);
+            }
         }
     }
 
     printk("Brought up %ld CPUs\n", (long)num_online_cpus());
+    if ( num_parked )
+        printk(XENLOG_INFO "Parked %u CPUs\n", num_parked);
     smp_cpus_done();
 
     do_initcalls();

xen/include/asm-x86/processor.h

@@ -390,12 +390,6 @@ static always_inline void set_in_cr4 (unsigned long mask)
     write_cr4(read_cr4() | mask);
 }
 
-static always_inline void clear_in_cr4 (unsigned long mask)
-{
-    mmu_cr4_features &= ~mask;
-    write_cr4(read_cr4() & ~mask);
-}
-
 /*
  *      NSC/Cyrix CPU configuration register indexes
  */