hvm: Fix get_immediate() in mmio decoder by reverting Dexuan Cui's

change that breaks the case mod==0, rm==4, sib&7==5. That is,
disp32(scaled-index).
Also simplify get_immediate with sign extension.
Signed-off-by: Keir Fraser <keir@xensource.com>

xen/arch/x86/hvm/platform.c

@@ -221,6 +221,7 @@ static inline unsigned long get_immediate(int ad_size, const unsigned char *inst
 
     inst++; //skip ModR/M byte
     if ( ad_size != WORD && mod != 3 && rm == 4 ) {
+        rm = *inst & 7;
         inst++; //skip SIB byte
     }
 
@@ -256,31 +257,15 @@ static inline unsigned long get_immediate(int ad_size, const unsigned char *inst
     return val;
 }
 
-/* Some instructions, like "add $imm8, r/m16"/"MOV $imm32, r/m64" require
- * the src immediate operand be sign-extented befere the op is executed. Here
- * we always sign-extend the operand to a "unsigned long" variable.
- *
- * Note: to simplify the logic here, the sign-extension here may be performed
- * redundantly against some instructions, like "MOV $imm16, r/m16" -- however
- * this is harmless, since we always remember the operand's size.
- */
-static inline unsigned long get_immediate_sign_ext(int ad_size,
-                                                   const unsigned char *inst,
-                                                   int op_size)
+static inline unsigned long get_immediate_sign_ext(
+    int ad_size, const unsigned char *inst, int op_size)
 {
     unsigned long result = get_immediate(ad_size, inst, op_size);
-
-    if ( op_size == QUAD )
-        op_size = LONG;
-
-    ASSERT( op_size == BYTE || op_size == WORD || op_size == LONG );
-
-    if ( result & (1UL << ((8*op_size) - 1)) )
-    {
-        unsigned long mask = ~0UL >> (8 * (sizeof(mask) - op_size));
-        result = ~mask | (result & mask);
-    }
-    return result;
+    if ( op_size == BYTE )
+        return (int8_t)result;
+    if ( op_size == WORD )
+        return (int16_t)result;
+    return (int32_t)result;
 }
 
 static inline int get_index(const unsigned char *inst, unsigned char rex)