x86: tolerate running on EFI runtime services page tables in map_domain_page()

In the event of a #PF while inan EFI runtime service function we
otherwise can't dump the page tables, making the analysis of the
problem more cumbersome.

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

--- a/xen/arch/x86/domain_page.c
+++ b/xen/arch/x86/domain_page.c
@@ -7,6 +7,7 @@
  */
 
 #include <xen/domain_page.h>
+#include <xen/efi.h>
 #include <xen/mm.h>
 #include <xen/perfc.h>
 #include <xen/pfn.h>
@@ -37,11 +38,14 @@ static inline struct vcpu *mapcache_curr
      */
     if ( unlikely(pagetable_is_null(v->arch.guest_table)) && is_pv_vcpu(v) )
     {
+        unsigned long cr3;
+
         /* If we really are idling, perform lazy context switch now. */
         if ( (v = idle_vcpu[smp_processor_id()]) == current )
             sync_local_execstate();
         /* We must now be running on the idle page table. */
-        ASSERT(read_cr3() == __pa(idle_pg_table));
+        ASSERT((cr3 = read_cr3()) == __pa(idle_pg_table) ||
+               (efi_enabled && cr3 == efi_rs_page_table()));
     }
 
     return v;
--- a/xen/arch/x86/efi/stub.c
+++ b/xen/arch/x86/efi/stub.c
@@ -9,6 +9,12 @@ const bool_t efi_enabled = 0;
 
 void __init efi_init_memory(void) { }
 
+paddr_t efi_rs_page_table(void)
+{
+    BUG();
+    return 0;
+}
+
 unsigned long efi_get_time(void)
 {
     BUG();
--- a/xen/common/efi/runtime.c
+++ b/xen/common/efi/runtime.c
@@ -98,6 +98,11 @@ void efi_rs_leave(unsigned long cr3)
     stts();
 }
 
+paddr_t efi_rs_page_table(void)
+{
+    return virt_to_maddr(efi_l4_pgtable);
+}
+
 unsigned long efi_get_time(void)
 {
     EFI_TIME time;
--- a/xen/include/xen/efi.h
+++ b/xen/include/xen/efi.h
@@ -28,6 +28,7 @@ struct xenpf_efi_runtime_call;
 struct compat_pf_efi_runtime_call;
 
 void efi_init_memory(void);
+paddr_t efi_rs_page_table(void);
 unsigned long efi_get_time(void);
 void efi_halt_system(void);
 void efi_reset_system(bool_t warm);