x86: small fixes to pcpu platform op handling

XENPF_get_cpuinfo should init the flags output field rather than only
modify it.

XENPF_cpu_online must check for the input CPU number to be in range.

XENPF_cpu_offline must also do that, and should also reject attempts to
offline CPU 0 (this fails in cpu_down() too, but preventing this here
appears more correct given that the code here calls
continue_hypercall_on_cpu(0, ...), which would be flawed if cpu_down()
would ever allow bringing down CPU 0 (and a distinct error code is
easier to deal with when debugging issues).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/platform_hypercall.c
+++ b/xen/arch/x86/platform_hypercall.c
@@ -449,13 +449,14 @@ ret_t do_platform_op(XEN_GUEST_HANDLE(xe
         if ( (g_info->xen_cpuid >= nr_cpu_ids) ||
              !cpu_present(g_info->xen_cpuid) )
         {
-            g_info->flags |= XEN_PCPU_FLAGS_INVALID;
+            g_info->flags = XEN_PCPU_FLAGS_INVALID;
         }
         else
         {
             g_info->apic_id = x86_cpu_to_apicid[g_info->xen_cpuid];
             g_info->acpi_id = acpi_get_processor_id(g_info->xen_cpuid);
             ASSERT(g_info->apic_id != BAD_APICID);
+            g_info->flags = 0;
             if (cpu_online(g_info->xen_cpuid))
                 g_info->flags |= XEN_PCPU_FLAGS_ONLINE;
         }
@@ -472,7 +473,7 @@ ret_t do_platform_op(XEN_GUEST_HANDLE(xe
     {
         int cpu = op->u.cpu_ol.cpuid;
 
-        if ( !cpu_present(cpu) )
+        if ( cpu >= nr_cpu_ids || !cpu_present(cpu) )
         {
             ret = -EINVAL;
             break;
@@ -493,7 +494,13 @@ ret_t do_platform_op(XEN_GUEST_HANDLE(xe
     {
         int cpu = op->u.cpu_ol.cpuid;
 
-        if ( !cpu_present(cpu) )
+        if ( cpu == 0 )
+        {
+            ret = -EOPNOTSUPP;
+            break;
+        }
+
+        if ( cpu >= nr_cpu_ids || !cpu_present(cpu) )
         {
             ret = -EINVAL;
             break;