[PATCH v2] x86/emul: Simplfy L{ES, DS, SS, FS, GS} handling

[PATCH v2] x86/emul: Simplfy L{ES, DS, SS, FS, GS} handling

[Andrew Cooper]

%ss, %fs and %gs can be calculated by directly masking the opcode.  %es and
%ds cant, but the calculation isn't hard.

Use seg rather than dst.val for storing the calculated segment, which is
appropriately typed.  Drop the sel local variable entirely and use dst.val
instead.  The mode_64bit() check can be repositioned and simplified to drop
the ext check.  Replace opencoding of X86EMUL_OKAY.

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

v2:
 * Drop the sel local variable entirely.
 * Move the lss/fs/gs cases back to their original location.
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 32 +++++++++++---------------------
 1 file changed, 11 insertions(+), 21 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index dfdcd6c..4753d0a 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -3397,24 +3397,20 @@ x86_emulate(
         _regs.eip = dst.val;
         break;
 
-    case 0xc4: /* les */ {
-        unsigned long sel;
-        dst.val = x86_seg_es;
-    les: /* dst.val identifies the segment */
-        generate_exception_if(mode_64bit() && !ext, EXC_UD);
+    case 0xc4: /* les */
+    case 0xc5: /* lds */
+        generate_exception_if(mode_64bit(), EXC_UD);
+        seg = (b & 1) * 3; /* es = 0, ds = 3 */
+    les:
         generate_exception_if(src.type != OP_MEM, EXC_UD);
         if ( (rc = read_ulong(src.mem.seg, src.mem.off + src.bytes,
-                              &sel, 2, ctxt, ops)) != 0 )
+                              &dst.val, 2, ctxt, ops)) != X86EMUL_OKAY )
             goto done;
-        if ( (rc = load_seg(dst.val, sel, 0, NULL, ctxt, ops)) != 0 )
+        ASSERT(is_x86_user_segment(seg));
+        if ( (rc = load_seg(seg, dst.val, 0, NULL, ctxt, ops)) != X86EMUL_OKAY )
             goto done;
         dst.val = src.val;
         break;
-    }
-
-    case 0xc5: /* lds */
-        dst.val = x86_seg_ds;
-        goto les;
 
     case 0xc8: /* enter imm16,imm8 */ {
         uint8_t depth = imm2 & 31;
@@ -5233,21 +5229,15 @@ x86_emulate(
         break;
 
     case X86EMUL_OPC(0x0f, 0xb2): /* lss */
-        dst.val = x86_seg_ss;
+    case X86EMUL_OPC(0x0f, 0xb4): /* lfs */
+    case X86EMUL_OPC(0x0f, 0xb5): /* lgs */
+        seg = b & 7;
         goto les;
 
     case X86EMUL_OPC(0x0f, 0xb3): btr: /* btr */
         emulate_2op_SrcV_nobyte("btr", src, dst, _regs.eflags);
         break;
 
-    case X86EMUL_OPC(0x0f, 0xb4): /* lfs */
-        dst.val = x86_seg_fs;
-        goto les;
-
-    case X86EMUL_OPC(0x0f, 0xb5): /* lgs */
-        dst.val = x86_seg_gs;
-        goto les;
-
     case X86EMUL_OPC(0x0f, 0xb6): /* movzx rm8,r{16,32,64} */
         /* Recompute DstReg as we may have decoded AH/BH/CH/DH. */
         dst.reg   = decode_register(modrm_reg, &_regs, 0);
-- 
2.1.4


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Re: [PATCH v2] x86/emul: Simplfy L{ES, DS, SS, FS, GS} handling

[Jan Beulich]

>>> On 15.12.16 at 18:10, <andrew.cooper3@citrix.com> wrote:
> %ss, %fs and %gs can be calculated by directly masking the opcode.  %es and
> %ds cant, but the calculation isn't hard.
> 
> Use seg rather than dst.val for storing the calculated segment, which is
> appropriately typed.  Drop the sel local variable entirely and use dst.val
> instead.  The mode_64bit() check can be repositioned and simplified to drop
> the ext check.  Replace opencoding of X86EMUL_OKAY.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

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


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