x86emul: special case far branch validation outside of long mode

In that case (with the new value being held in, or now in one case cast
to, a 32-bit variable) there's no need to go through the long mode part
of the checks.

Primarily this was meant to hopefully address Coverity ID 1355278, but
since the change produces smaller code as well I think we should use it
even if it doesn't help that (benign) warning.

Also it's more in line with jmp_rel() for commit_far_branch() to do the
_regs.eip update, so adjust that at once.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -652,13 +652,20 @@ do {
     _regs.eip = ip;                                                     \
 } while (0)
 
-#define validate_far_branch(cs, ip)                                     \
-    generate_exception_if(in_longmode(ctxt, ops) && (cs)->attr.fields.l \
-                          ? !is_canonical_address(ip)                   \
-                          : (ip) > (cs)->limit, EXC_GP, 0)
+#define validate_far_branch(cs, ip) ({                                  \
+    if ( sizeof(ip) <= 4 ) {                                            \
+        ASSERT(in_longmode(ctxt, ops) <= 0);                            \
+        generate_exception_if((ip) > (cs)->limit, EXC_GP, 0);           \
+    } else                                                              \
+        generate_exception_if(in_longmode(ctxt, ops) &&                 \
+                              (cs)->attr.fields.l                       \
+                              ? !is_canonical_address(ip)               \
+                              : (ip) > (cs)->limit, EXC_GP, 0);         \
+})
 
 #define commit_far_branch(cs, ip) ({                                    \
     validate_far_branch(cs, ip);                                        \
+    _regs.eip = (ip);                                                   \
     ops->write_segment(x86_seg_cs, cs, ctxt);                           \
 })
 
@@ -2787,7 +2794,6 @@ x86_emulate(
              (rc = load_seg(x86_seg_cs, src.val, 1, &cs, ctxt, ops)) ||
              (rc = commit_far_branch(&cs, dst.val)) )
             goto done;
-        _regs.eip = dst.val;
         break;
     }
 
@@ -2830,9 +2836,8 @@ x86_emulate(
         eflags &= 0x257fd5;
         _regs.eflags &= mask;
         _regs.eflags |= (eflags & ~mask) | 0x02;
-        _regs.eip = eip;
         if ( (rc = load_seg(x86_seg_cs, sel, 1, &cs, ctxt, ops)) ||
-             (rc = commit_far_branch(&cs, eip)) )
+             (rc = commit_far_branch(&cs, (uint32_t)eip)) )
             goto done;
         break;
     }
@@ -3465,7 +3470,6 @@ x86_emulate(
         if ( (rc = load_seg(x86_seg_cs, sel, 0, &cs, ctxt, ops)) ||
              (rc = commit_far_branch(&cs, eip)) )
             goto done;
-        _regs.eip = eip;
         break;
     }
 
@@ -3767,11 +3771,11 @@ x86_emulate(
                                       &_regs.eip, op_bytes, ctxt)) ||
                      (rc = ops->write_segment(x86_seg_cs, &cs, ctxt)) )
                     goto done;
+                _regs.eip = src.val;
             }
             else if ( (rc = load_seg(x86_seg_cs, sel, 0, &cs, ctxt, ops)) ||
                       (rc = commit_far_branch(&cs, src.val)) )
                 goto done;
-            _regs.eip = src.val;
 
             dst.type = OP_NONE;
             break;