x86/traps: export the exception_table[] function pointer table to C

and use it in preference to the direct call to do_nmi() in vmx.c

The value 'TRAP_last_reserved' was only used where 'TRAP_nr' would be more
appropriate, so is replaced.

No functional change

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/vmx/vmx.c

@@ -2701,7 +2701,7 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
              && ((intr_info & INTR_INFO_INTR_TYPE_MASK) ==
                  (X86_EVENTTYPE_NMI << 8)) )
         {
-            do_nmi(regs);
+            exception_table[TRAP_nmi](regs);
             enable_nmis();
         }
         break;

xen/arch/x86/x86_64/entry.S

@@ -728,9 +728,10 @@ ENTRY(exception_table)
         .quad do_alignment_check
         .quad do_machine_check
         .quad do_simd_coprocessor_error
-        .rept TRAP_last_reserved + 1 - ((. - exception_table) / 8)
+        .rept TRAP_nr - ((. - exception_table) / 8)
         .quad do_reserved_trap /* Architecturally reserved exceptions. */
         .endr
+        .size exception_table, . - exception_table
 
 ENTRY(hypercall_table)
         .quad do_set_trap_table     /*  0 */
@@ -857,7 +858,7 @@ autogen_stubs: /* Automatically generated stubs. */
         entrypoint 1b
 
         /* Reserved exceptions, heading towards do_reserved_trap(). */
-        .elseif vec == TRAP_copro_seg || vec == TRAP_spurious_int || (vec > TRAP_simd_error && vec <= TRAP_last_reserved)
+        .elseif vec == TRAP_copro_seg || vec == TRAP_spurious_int || (vec > TRAP_simd_error && vec < TRAP_nr)
 
 1:      test  $8,%spl        /* 64bit exception frames are 16 byte aligned, but the word */
         jz    2f             /* size is 8 bytes.  Check whether the processor gave us an */

xen/include/asm-x86/processor.h

@@ -114,7 +114,7 @@
 #define TRAP_machine_check    18
 #define TRAP_simd_error       19
 #define TRAP_virtualisation   20
-#define TRAP_last_reserved    31
+#define TRAP_nr               32
 
 /* Set for entry via SYSCALL. Informs return code to use SYSRETQ not IRETQ. */
 /* NB. Same as VGCF_in_syscall. No bits in common with any other TRAP_ defn. */
@@ -492,6 +492,9 @@ extern void mtrr_bp_init(void);
 
 void mcheck_init(struct cpuinfo_x86 *c, bool_t bsp);
 
+/* Dispatch table for exceptions */
+extern void (* const exception_table[TRAP_nr])(struct cpu_user_regs *regs);
+
 #define DECLARE_TRAP_HANDLER(_name)                    \
     void _name(void);                                  \
     void do_ ## _name(struct cpu_user_regs *regs)