xen/arm: Allow setting the number of CPUs to activate at runtime

Introduce a command line parameter "maxcpus" on Arm to allow adjusting
the number of CPUs to activate. Currently the limit is defined by the
config option CONFIG_NR_CPUS. Such parameter already exists on x86.

Define a parameter "maxcpus" and a corresponding static variable
max_cpus in Arm smpboot.c. Modify function smp_get_max_cpus to take
max_cpus as a limit and to return proper unsigned int instead of int.

Take the opportunity to remove redundant variable cpus from start_xen
function and to directly assign the return value from smp_get_max_cpus
to nr_cpu_ids (global variable in Xen used to store the number of CPUs
actually activated).

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

docs/misc/xen-command-line.pandoc

@@ -1652,13 +1652,16 @@ with **crashinfo_maxaddr**.
 Specify the threshold below which Xen will inform dom0 that the quantity of
 free memory is getting low.  Specifying `0` will disable this notification.
 
-### maxcpus (x86)
+### maxcpus
 > `= <integer>`
 
 Specify the maximum number of CPUs that should be brought up.
 
 This option is ignored in **pv-shim** mode.
 
+**WARNING: On Arm big.LITTLE systems, when `hmp-unsafe` option is enabled, this command line
+option does not guarantee on which CPU types will be used.**
+
 ### max_cstate (x86)
 > `= <integer>[,<integer>]`
 

xen/arch/arm/include/asm/smp.h

@@ -33,7 +33,7 @@ extern void init_secondary(void);
 
 extern void smp_init_cpus(void);
 extern void smp_clear_cpu_maps (void);
-extern int smp_get_max_cpus (void);
+extern unsigned int smp_get_max_cpus(void);
 
 #define cpu_physical_id(cpu) cpu_logical_map(cpu)
 

xen/arch/arm/setup.c

@@ -871,11 +871,10 @@ void __init start_xen(unsigned long boot_phys_offset,
                       unsigned long fdt_paddr)
 {
     size_t fdt_size;
-    int cpus, i;
     const char *cmdline;
     struct bootmodule *xen_bootmodule;
     struct domain *d;
-    int rc;
+    int rc, i;
 
     dcache_line_bytes = read_dcache_line_bytes();
 
@@ -951,9 +950,8 @@ void __init start_xen(unsigned long boot_phys_offset,
     processor_id();
 
     smp_init_cpus();
-    cpus = smp_get_max_cpus();
-    printk(XENLOG_INFO "SMP: Allowing %u CPUs\n", cpus);
-    nr_cpu_ids = cpus;
+    nr_cpu_ids = smp_get_max_cpus();
+    printk(XENLOG_INFO "SMP: Allowing %u CPUs\n", nr_cpu_ids);
 
     /*
      * Some errata relies on SMCCC version which is detected by psci_init()
@@ -997,7 +995,7 @@ void __init start_xen(unsigned long boot_phys_offset,
 
     for_each_present_cpu ( i )
     {
-        if ( (num_online_cpus() < cpus) && !cpu_online(i) )
+        if ( (num_online_cpus() < nr_cpu_ids) && !cpu_online(i) )
         {
             int ret = cpu_up(i);
             if ( ret != 0 )

xen/arch/arm/smpboot.c

@@ -43,6 +43,10 @@ cpumask_t cpu_possible_map;
 
 struct cpuinfo_arm cpu_data[NR_CPUS];
 
+/* maxcpus: maximum number of CPUs to activate. */
+static unsigned int __initdata max_cpus;
+integer_param("maxcpus", max_cpus);
+
 /* CPU logical map: map xen cpuid to an MPIDR */
 register_t __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1] = MPIDR_INVALID };
 
@@ -277,16 +281,18 @@ void __init smp_init_cpus(void)
                     "unless the cpu affinity of all domains is specified.\n");
 }
 
-int __init
-smp_get_max_cpus (void)
+unsigned int __init smp_get_max_cpus(void)
 {
-    int i, max_cpus = 0;
+    unsigned int i, cpus = 0;
 
-    for ( i = 0; i < nr_cpu_ids; i++ )
+    if ( ( !max_cpus ) || ( max_cpus > nr_cpu_ids ) )
+        max_cpus = nr_cpu_ids;
+
+    for ( i = 0; i < max_cpus; i++ )
         if ( cpu_possible(i) )
-            max_cpus++;
+            cpus++;
 
-    return max_cpus;
+    return cpus;
 }
 
 void __init