x86/mm: avoid inadvertently degrading a TLB flush to local only

If the direct map is incorrectly modified with interrupts disabled,
the required TLB flushes are degraded to flushing the local CPU only.

This could lead to very hard to diagnose problems as different CPUs will
end up with different views of memory. Although, no such issues have yet
been identified.

Change the check in the flush_area() macro to look at system_state
instead. This defers the switch from local to all later in the boot
(see xen/arch/x86/setup.c:__start_xen()). This is fine because
additional PCPUs are not brought up until after the system state is
SYS_STATE_smp_boot.

Signed-off-by: David Vrabel <dvrabel@amazon.co.uk>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm.c

@@ -5071,11 +5071,10 @@ l1_pgentry_t *virt_to_xen_l1e(unsigned long v)
 #define lNf_to_l1f(f) (((f) & _PAGE_PRESENT) ? ((f) & ~_PAGE_PSE) : (f))
 
 /*
- * map_pages_to_xen() can be called with interrupts disabled during
- * early bootstrap. In this case it is safe to use flush_area_local()
- * and avoid locking because only the local CPU is online.
+ * map_pages_to_xen() can be called early in boot before any other
+ * CPUs are online. Use flush_area_local() in this case.
  */
-#define flush_area(v,f) (!local_irq_is_enabled() ?              \
+#define flush_area(v,f) (system_state < SYS_STATE_smp_boot ?    \
                          flush_area_local((const void *)v, f) : \
                          flush_area_all((const void *)v, f))