Even not-so-recent Linux may, due to post-2.6.18 changes to the
process creation code, cause quite a number (depending on environment
and argument size) of faulting accesses to user space originating from
kernel mode. Generally those happen for non-present pages and would
lead to a nested page fault from guest_get_eff_l1e(). They can be
avoided by checking for PFEC_page_present as long as the guest isn't
running on shadow page tables.

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

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1244,13 +1244,19 @@ static int fixup_page_fault(unsigned lon
     }
 
     if ( VM_ASSIST(d, VMASST_TYPE_writable_pagetables) &&
-         guest_kernel_mode(v, regs) &&
-         /* Do not check if access-protection fault since the page may 
-            legitimately be not present in shadow page tables */
-         ((regs->error_code & (PFEC_write_access|PFEC_reserved_bit)) ==
-          PFEC_write_access) &&
-         ptwr_do_page_fault(v, addr, regs) )
-        return EXCRET_fault_fixed;
+         guest_kernel_mode(v, regs) )
+    {
+        unsigned int mbs = PFEC_write_access;
+
+        /* Do not check if access-protection fault since the page may 
+           legitimately be not present in shadow page tables */
+        if ( !paging_mode_enabled(d) )
+            mbs |= PFEC_page_present;
+        if ( (regs->error_code & (mbs | PFEC_reserved_bit |
+                                  PFEC_insn_fetch)) == mbs &&
+             ptwr_do_page_fault(v, addr, regs) )
+            return EXCRET_fault_fixed;
+    }
 
     /* For non-external shadowed guests, we fix up both their own 
      * pagefaults and Xen's, since they share the pagetables. */