x86: introduce x86_seg_sys

To represent the USER-MSR bitmap access, a new segment type needs
introducing, behaving like x86_seg_none in terms of address treatment,
but behaving like a system segment for page walk purposes (implicit
supervisor-mode access).

While there also add x86_seg_none handling to the test harness'es
read() hook, as will be needed for MSR-LIST support.

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

tools/tests/x86_emulator/test_x86_emulator.c

@@ -593,6 +593,8 @@ static int read(
     default:
         if ( !is_x86_user_segment(seg) )
             return X86EMUL_UNHANDLEABLE;
+    case x86_seg_sys:
+    case x86_seg_none:
         bytes_read += bytes;
         break;
     }

xen/arch/x86/hvm/emulate.c

@@ -840,7 +840,7 @@ static int hvmemul_virtual_to_linear(
     int okay;
     unsigned long reps = 1;
 
-    if ( seg == x86_seg_none )
+    if ( seg == x86_seg_none || seg == x86_seg_sys )
     {
         *linear = offset;
         return X86EMUL_OKAY;

xen/arch/x86/hvm/hvm.c

@@ -2599,7 +2599,7 @@ bool hvm_vcpu_virtual_to_linear(
      * It is expected that the access rights of reg are suitable for seg (and
      * that this is enforced at the point that seg is loaded).
      */
-    ASSERT(seg < x86_seg_none);
+    ASSERT(seg < x86_seg_sys);
 
     /* However, check that insn fetches only ever specify CS. */
     ASSERT(access_type != hvm_access_insn_fetch || seg == x86_seg_cs);

xen/arch/x86/hvm/svm/svm.c

@@ -749,6 +749,7 @@ static void cf_check svm_set_segment_register(
         vmcb->ldtr = *reg;
         break;
 
+    case x86_seg_sys:
     case x86_seg_none:
         ASSERT_UNREACHABLE();
         break;

xen/arch/x86/x86_emulate/x86_emulate.h

@@ -43,7 +43,8 @@ enum x86_segment {
     x86_seg_ldtr,
     x86_seg_gdtr,
     x86_seg_idtr,
-    /* No Segment: For accesses which are already linear. */
+    /* No Segment: For (system/normal) accesses which are already linear. */
+    x86_seg_sys,
     x86_seg_none
 };