[Xen-devel] [PATCH] x86/emul: Reorder the user segments in x86_segment to match SReg3 encoding

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

This avoids needing a translation table between hardware ordering and Xen's
ordering.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 35 +++++++++++++++-------------------
 xen/arch/x86/x86_emulate/x86_emulate.h |  4 ++--
 2 files changed, 17 insertions(+), 22 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 38147c5..32c45c4 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1557,22 +1557,6 @@ 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;
-    }
-    return decode_segment_failed;
-}
-
 static bool is_aligned(enum x86_segment seg, unsigned long offs,
                        unsigned int size, struct x86_emulate_ctxt *ctxt,
                        const struct x86_emulate_ops *ops)
@@ -2980,8 +2964,8 @@ x86_emulate(
         break;
 
     case 0x8c: /* mov Sreg,r/m */
-        seg = decode_segment(modrm_reg);
-        generate_exception_if(seg == decode_segment_failed, EXC_UD, -1);
+        seg = modrm_reg;
+        generate_exception_if(!is_x86_user_segment(seg), EXC_UD, -1);
     store_selector:
         fail_if(ops->read_segment == NULL);
         if ( (rc = ops->read_segment(seg, &sreg, ctxt)) != 0 )
@@ -2992,8 +2976,8 @@ x86_emulate(
         break;
 
     case 0x8e: /* mov r/m,Sreg */
-        seg = decode_segment(modrm_reg);
-        generate_exception_if(seg == decode_segment_failed, EXC_UD, -1);
+        seg = modrm_reg;
+        generate_exception_if(!is_x86_user_segment(seg), EXC_UD, -1);
         generate_exception_if(seg == x86_seg_cs, EXC_UD, -1);
         if ( (rc = load_seg(seg, src.val, 0, NULL, ctxt, ops)) != 0 )
             goto done;
@@ -5520,4 +5504,15 @@ x86_insn_length(const struct x86_emulate_state *state,
     return state->eip - ctxt->regs->eip;
 }
 
+static void __init __maybe_unused 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);
+}
+
 #endif
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.h 
b/xen/arch/x86/x86_emulate/x86_emulate.h
index 641711e..00ceade 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -29,11 +29,11 @@ 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. */
-- 
2.1.4


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