x86/traps: honor EXT bit in error codes

The specification does not explicitly limit the use of this bit to
exceptions that can have selector style error codes, so to be on the
safe side we should deal with it being set even on error codes formally
documented to be always zero (if they're indeed always zero, the change
is simply dead code in those cases).

Introduce and use (where suitable) X86_XEC_* constants to make the code
easier to read.

To match the placement of the "hardware_trap" label, the "hardware_gp"
one gets moved slightly too.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Drop #MC related changes. Simplify initial condition added to
    do_trap(). Adjust a comment in do_general_protection().

--- unstable.orig/xen/arch/x86/traps.c	2015-10-13 12:52:27.000000000 +0200
+++ unstable/xen/arch/x86/traps.c	2015-11-11 09:51:34.000000000 +0100
@@ -618,6 +618,9 @@ static void do_trap(struct cpu_user_regs
     unsigned int trapnr = regs->entry_vector;
     unsigned long fixup;
 
+    if ( regs->error_code & X86_XEC_EXT )
+        goto hardware_trap;
+
     DEBUGGER_trap_entry(trapnr, regs);
 
     if ( guest_mode(regs) )
@@ -644,6 +647,7 @@ static void do_trap(struct cpu_user_regs
         return;
     }
 
+ hardware_trap:
     DEBUGGER_trap_fatal(trapnr, regs);
 
     show_execution_state(regs);
@@ -1265,13 +1269,14 @@ static int handle_gdt_ldt_mapping_fault(
             tb = propagate_page_fault(curr->arch.pv_vcpu.ldt_base + offset,
                                       regs->error_code);
             if ( tb )
-                tb->error_code = ((u16)offset & ~3) | 4;
+                tb->error_code = (offset & ~(X86_XEC_EXT | X86_XEC_IDT)) |
+                                 X86_XEC_TI;
         }
     }
     else
     {
         /* GDT fault: handle the fault as #GP(selector). */
-        regs->error_code = (u16)offset & ~7;
+        regs->error_code = offset & ~(X86_XEC_EXT | X86_XEC_IDT | X86_XEC_TI);
         (void)do_general_protection(regs);
     }
 
@@ -3231,7 +3236,7 @@ void do_general_protection(struct cpu_us
 
     DEBUGGER_trap_entry(TRAP_gp_fault, regs);
 
-    if ( regs->error_code & 1 )
+    if ( regs->error_code & X86_XEC_EXT )
         goto hardware_gp;
 
     if ( !guest_mode(regs) )
@@ -3250,14 +3255,15 @@ void do_general_protection(struct cpu_us
      * 
      * Instead, a GPF occurs with the faulting IDT vector in the error code.
      * Bit 1 is set to indicate that an IDT entry caused the fault. Bit 0 is 
-     * clear to indicate that it's a software fault, not hardware.
+     * clear (which got already checked above) to indicate that it's a software
+     * fault, not a hardware one.
      * 
      * NOTE: Vectors 3 and 4 are dealt with from their own handler. This is
      * okay because they can only be triggered by an explicit DPL-checked
      * instruction. The DPL specified by the guest OS for these vectors is NOT
      * CHECKED!!
      */
-    if ( (regs->error_code & 3) == 2 )
+    if ( regs->error_code & X86_XEC_IDT )
     {
         /* This fault must be due to <INT n> instruction. */
         const struct trap_info *ti;
@@ -3299,9 +3305,9 @@ void do_general_protection(struct cpu_us
         return;
     }
 
+ hardware_gp:
     DEBUGGER_trap_fatal(TRAP_gp_fault, regs);
 
- hardware_gp:
     show_execution_state(regs);
     panic("GENERAL PROTECTION FAULT\n[error_code=%04x]", regs->error_code);
 }
--- unstable.orig/xen/arch/x86/x86_64/entry.S	2015-11-11 09:45:03.000000000 +0100
+++ unstable/xen/arch/x86/x86_64/entry.S	2015-11-04 09:18:27.000000000 +0100
@@ -338,7 +338,7 @@ int80_slow_path:
          * Setup entry vector and error code as if this was a GPF caused by an
          * IDT entry with DPL==0.
          */
-        movl  $((0x80 << 3) | 0x2),UREGS_error_code(%rsp)
+        movl  $((0x80 << 3) | X86_XEC_IDT),UREGS_error_code(%rsp)
         SAVE_PRESERVED
         movl  $TRAP_gp_fault,UREGS_entry_vector(%rsp)
         /* A GPF wouldn't have incremented the instruction pointer. */
--- unstable.orig/xen/include/asm-x86/processor.h	2015-11-11 09:45:03.000000000 +0100
+++ unstable/xen/include/asm-x86/processor.h	2015-11-04 09:21:34.000000000 +0100
@@ -143,6 +143,11 @@
 #define PFEC_page_paged     (1U<<5)
 #define PFEC_page_shared    (1U<<6)
 
+/* Other exception error code values. */
+#define X86_XEC_EXT         (_AC(1,U) << 0)
+#define X86_XEC_IDT         (_AC(1,U) << 1)
+#define X86_XEC_TI          (_AC(1,U) << 2)
+
 #define XEN_MINIMAL_CR4 (X86_CR4_PGE | X86_CR4_PAE)
 
 #define XEN_SYSCALL_MASK (X86_EFLAGS_AC|X86_EFLAGS_VM|X86_EFLAGS_RF|    \