# Commit 0888d36bb23f7365ce12b03127fd0fb2661ec90e
# Date 2016-10-26 14:04:12 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/emul: Correct the decoding of SReg3 operands

REX.R is ignored when considering segment register operands, and needs masking
out first.

While fixing this, reorder the user segments in x86_segment to match SReg3
encoding.  This avoids needing a translation table between hardware ordering
and Xen's ordering.

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

# Commit a62511bf14971ff581212decbbf57fc11b967840
# Date 2016-10-31 08:57:47 +0100
# Author Jan Beulich <jbeulich@suse.com>
# Committer Jan Beulich <jbeulich@suse.com>
VMX: fix realmode emulation SReg handling

Commit 0888d36bb2 ("x86/emul: Correct the decoding of SReg3 operands")
overlooked three places where x86_seg_cs was assumed to be zero.

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

--- a/tools/tests/x86_emulator/x86_emulate.c
+++ b/tools/tests/x86_emulator/x86_emulate.c
@@ -13,6 +13,16 @@ typedef bool bool_t;
 #define BUG() abort()
 #define ASSERT assert
 
+#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
+/* Force a compilation error if condition is true */
+#define BUILD_BUG_ON(cond) ({ _Static_assert(!(cond), "!(" #cond ")"); })
+#define BUILD_BUG_ON_ZERO(cond) \
+    sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
+#else
+#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
+#endif
+
 #define cpu_has_amd_erratum(nr) 0
 #define mark_regs_dirty(r) ((void)(r))
 
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1405,23 +1405,6 @@ decode_register(
     return p;
 }
 
-#define decode_segment_failed x86_seg_tr
-static enum x86_segment
-decode_segment(uint8_t modrm_reg)
-{
-    switch ( modrm_reg )
-    {
-    case 0: return x86_seg_es;
-    case 1: return x86_seg_cs;
-    case 2: return x86_seg_ss;
-    case 3: return x86_seg_ds;
-    case 4: return x86_seg_fs;
-    case 5: return x86_seg_gs;
-    default: break;
-    }
-    return decode_segment_failed;
-}
-
 /* Inject a software interrupt/exception, emulating if needed. */
 static int inject_swint(enum x86_swint_type type,
                         uint8_t vector, uint8_t insn_len,
@@ -2439,8 +2422,8 @@ x86_emulate(
 
     case 0x8c: /* mov Sreg,r/m */ {
         struct segment_register reg;
-        enum x86_segment seg = decode_segment(modrm_reg);
-        generate_exception_if(seg == decode_segment_failed, EXC_UD, -1);
+        enum x86_segment seg = modrm_reg & 7; /* REX.R is ignored. */
+        generate_exception_if(!is_x86_user_segment(seg), EXC_UD, -1);
         fail_if(ops->read_segment == NULL);
         if ( (rc = ops->read_segment(seg, &reg, ctxt)) != 0 )
             goto done;
@@ -2451,9 +2434,9 @@ x86_emulate(
     }
 
     case 0x8e: /* mov r/m,Sreg */ {
-        enum x86_segment seg = decode_segment(modrm_reg);
-        generate_exception_if(seg == decode_segment_failed, EXC_UD, -1);
-        generate_exception_if(seg == x86_seg_cs, EXC_UD, -1);
+        enum x86_segment seg = modrm_reg & 7; /* REX.R is ignored. */
+        generate_exception_if(!is_x86_user_segment(seg) ||
+                              seg == x86_seg_cs, EXC_UD, -1);
         if ( (rc = load_seg(seg, src.val, 0, NULL, ctxt, ops)) != 0 )
             goto done;
         if ( seg == x86_seg_ss )
@@ -4747,3 +4730,14 @@ x86_emulate(
  cannot_emulate:
     return X86EMUL_UNHANDLEABLE;
 }
+
+static inline void build_assertions(void)
+{
+    /* Check the values against SReg3 encoding in opcode/ModRM bytes. */
+    BUILD_BUG_ON(x86_seg_es != 0);
+    BUILD_BUG_ON(x86_seg_cs != 1);
+    BUILD_BUG_ON(x86_seg_ss != 2);
+    BUILD_BUG_ON(x86_seg_ds != 3);
+    BUILD_BUG_ON(x86_seg_fs != 4);
+    BUILD_BUG_ON(x86_seg_gs != 5);
+}
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1254,21 +1254,23 @@ static void vmx_update_guest_cr(struct v
             enum x86_segment s; 
             struct segment_register reg[x86_seg_tr + 1];
 
+            BUILD_BUG_ON(x86_seg_tr != x86_seg_gs + 1);
+
             /* Entering or leaving real mode: adjust the segment registers.
              * Need to read them all either way, as realmode reads can update
              * the saved values we'll use when returning to prot mode. */
-            for ( s = x86_seg_cs ; s <= x86_seg_tr ; s++ )
+            for ( s = 0; s < ARRAY_SIZE(reg); s++ )
                 vmx_get_segment_register(v, s, &reg[s]);
             v->arch.hvm_vmx.vmx_realmode = realmode;
             
             if ( realmode )
             {
-                for ( s = x86_seg_cs ; s <= x86_seg_tr ; s++ )
+                for ( s = 0; s < ARRAY_SIZE(reg); s++ )
                     vmx_set_segment_register(v, s, &reg[s]);
             }
             else 
             {
-                for ( s = x86_seg_cs ; s <= x86_seg_tr ; s++ ) 
+                for ( s = 0; s < ARRAY_SIZE(reg); s++ )
                     if ( !(v->arch.hvm_vmx.vm86_segment_mask & (1<<s)) )
                         vmx_set_segment_register(
                             v, s, &v->arch.hvm_vmx.vm86_saved_seg[s]);
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -28,11 +28,11 @@ struct x86_emulate_ctxt;
 
 /* Comprehensive enumeration of x86 segment registers. */
 enum x86_segment {
-    /* General purpose. */
+    /* General purpose.  Matches the SReg3 encoding in opcode/ModRM bytes. */
+    x86_seg_es,
     x86_seg_cs,
     x86_seg_ss,
     x86_seg_ds,
-    x86_seg_es,
     x86_seg_fs,
     x86_seg_gs,
     /* System. */
