x86: use / "support" UDB

With opcode D6h now firmly reserved as another #UD-raising one in 64-bit
mode, use that instead of the two-byte UD2 for bug frame marking.

While there also make a minor adjustment to the emulator.

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

xen/arch/x86/include/asm/bug.h

@@ -21,7 +21,7 @@
 
 #ifndef __ASSEMBLER__
 
-#define BUG_INSTR       "ud2"
+#define BUG_INSTR       ".byte 0xd6" /* UDB, requiring gas 2.46 */
 #define BUG_ASM_CONST   "c"
 
 #else  /* !__ASSEMBLER__ */
@@ -37,7 +37,7 @@
         .error "Invalid BUGFRAME index"
     .endif
 
-    .L\@ud: ud2a
+    .L\@ud: .byte 0xd6 /* UDB, requiring gas 2.46 */
 
     .pushsection .rodata.str1, "aMS", @progbits, 1
          .L\@s1: .asciz "\file_str"

xen/arch/x86/traps.c

@@ -1338,7 +1338,7 @@ void asmlinkage do_trap(struct cpu_user_regs *regs)
 
 void asmlinkage do_invalid_op(struct cpu_user_regs *regs)
 {
-    u8 bug_insn[2];
+    uint8_t bug_insn;
     const void *eip = (const void *)regs->rip;
     int id;
 
@@ -1350,8 +1350,8 @@ void asmlinkage do_invalid_op(struct cpu_user_regs *regs)
     }
 
     if ( !is_active_kernel_text(regs->rip) ||
-         copy_from_unsafe(bug_insn, eip, sizeof(bug_insn)) ||
-         memcmp(bug_insn, "\xf\xb", sizeof(bug_insn)) )
+         copy_from_unsafe(&bug_insn, eip, sizeof(bug_insn)) ||
+         bug_insn != 0xd6 /* UDB */ )
         goto die;
 
     id = do_bug_frame(regs, regs->rip);

xen/arch/x86/x86_emulate/decode.c

@@ -652,7 +652,7 @@ decode_onebyte(struct x86_emulate_state *s,
     case 0xce: /* into */
     case 0xd4: /* aam */
     case 0xd5: /* aad */
-    case 0xd6: /* salc */
+    case 0xd6: /* salc / udb */
         s->not_64bit = true;
         break;