arm/xen: fix SMP guests boot

Commit 88e957d6 ("xen: introduce xen_vcpu_id mapping") broke SMP
ARM guests on Xen. When FIFO-based event channels are in use (this is
the default), evtchn_fifo_alloc_control_block() is called on
CPU_UP_PREPARE event and this happens before we set up xen_vcpu_id
mapping in xen_starting_cpu. Temporary fix the issue by setting direct
Linux CPU id <-> Xen vCPU id mapping for all possible CPUs at boot. We
don't currently support kexec/kdump on Xen/ARM so these ids always
match.

In future, we have several ways to solve the issue, e.g.:

- Eliminate all hypercalls from CPU_UP_PREPARE, do them from the
  starting CPU. This can probably be done for both x86 and ARM and, if
  done, will allow us to get Xen's idea of vCPU id from CPUID/MPIDR on
  the starting CPU directly, no messing with ACPI/device tree
  required.

- Save vCPU id information from ACPI/device tree on ARM and use it to
  initialize xen_vcpu_id mapping. This is the same trick we currently
  do on x86.
Reported-by: Julien Grall <julien.grall@arm.com>
Tested-by: Wei Chen <Wei.Chen@arm.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/arm/xen/enlighten.c

@@ -170,9 +170,6 @@ static int xen_starting_cpu(unsigned int cpu)
 	pr_info("Xen: initializing cpu%d\n", cpu);
 	vcpup = per_cpu_ptr(xen_vcpu_info, cpu);
 
-	/* Direct vCPU id mapping for ARM guests. */
-	per_cpu(xen_vcpu_id, cpu) = cpu;
-
 	info.mfn = virt_to_gfn(vcpup);
 	info.offset = xen_offset_in_page(vcpup);
 
@@ -330,6 +327,7 @@ static int __init xen_guest_init(void)
 {
 	struct xen_add_to_physmap xatp;
 	struct shared_info *shared_info_page = NULL;
+	int cpu;
 
 	if (!xen_domain())
 		return 0;
@@ -380,7 +378,8 @@ static int __init xen_guest_init(void)
 		return -ENOMEM;
 
 	/* Direct vCPU id mapping for ARM guests. */
-	per_cpu(xen_vcpu_id, 0) = 0;
+	for_each_possible_cpu(cpu)
+		per_cpu(xen_vcpu_id, cpu) = cpu;
 
 	xen_auto_xlat_grant_frames.count = gnttab_max_grant_frames();
 	if (xen_xlate_map_ballooned_pages(&xen_auto_xlat_grant_frames.pfn,