[PATCH] arm/xen: fix SMP guests boot

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Commit 88e957d6e47f ("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>
---
It would be nice if this patch could still make it to 4.8 as all SMP
ARM/Xen guests are currently broken.
---
 arch/arm/xen/enlighten.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
index 3d2cef6..f193414 100644
--- a/arch/arm/xen/enlighten.c
+++ b/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,
-- 
2.7.4