xen/arm: do not setup the runstate info page if kpti is enabled

The VCPUOP_register_runstate_memory_area hypercall takes a virtual
address of a buffer as a parameter. The semantics of the hypercall are
such that the virtual address should always be valid.

When KPTI is enabled and we are running userspace code, the virtual
address is not valid, thus, Linux is violating the semantics of
VCPUOP_register_runstate_memory_area.

Do not call VCPUOP_register_runstate_memory_area when KPTI is enabled.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
CC: Bertrand Marquis <Bertrand.Marquis@arm.com>
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
Link: https://lore.kernel.org/r/20200924234955.15455-1-sstabellini@kernel.org
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

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

@@ -1 +1,6 @@
 #include <xen/arm/page.h>
+
+static inline bool xen_kernel_unmapped_at_usr(void)
+{
+	return false;
+}

arch/arm/xen/enlighten.c

@@ -158,7 +158,8 @@ static int xen_starting_cpu(unsigned int cpu)
 	BUG_ON(err);
 	per_cpu(xen_vcpu, cpu) = vcpup;
 
-	xen_setup_runstate_info(cpu);
+	if (!xen_kernel_unmapped_at_usr())
+		xen_setup_runstate_info(cpu);
 
 after_register_vcpu_info:
 	enable_percpu_irq(xen_events_irq, 0);
@@ -387,7 +388,8 @@ static int __init xen_guest_init(void)
 		return -EINVAL;
 	}
 
-	xen_time_setup_guest();
+	if (!xen_kernel_unmapped_at_usr())
+		xen_time_setup_guest();
 
 	if (xen_initial_domain())
 		pvclock_gtod_register_notifier(&xen_pvclock_gtod_notifier);

arch/arm64/include/asm/xen/page.h

@@ -1 +1,7 @@
 #include <xen/arm/page.h>
+#include <asm/mmu.h>
+
+static inline bool xen_kernel_unmapped_at_usr(void)
+{
+	return arm64_kernel_unmapped_at_el0();
+}