[PATCH] x86emul: avoid double memory read for RORX

[Jan Beulich <jbeulich@xxxxxxxx>]

Originally only twobyte_table[0x3a] determined what part of generic
operand fetching (near the top of x86_emulate()) comes into play. When
ext0f3a_table[] was added, ->desc was updated to properly describe the
ModR/M byte's function. With that generic source operand fetching came
into play for RORX, rendering the explicit fetching in the respective
case block redundant (and wrong at the very least when MMIO with side
effects is accessed).

While there also make a purely cosmetic / documentary adjustment to
ext0f3a_table[]: RORX really is a 2-operand insn, MOV-like in that it
only writes its destination register.

Fixes: 9f7f5f6bc95b ("x86emul: add tables for 0f38 and 0f3a extension space")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/decode.c
+++ b/xen/arch/x86/x86_emulate/decode.c
@@ -521,7 +521,7 @@ static const struct ext0f3a_table {
     [0xce ... 0xcf] = { .simd_size = simd_packed_int, .d8s = d8s_vl },
     [0xde] = { .simd_size = simd_other },
     [0xdf] = { .simd_size = simd_packed_int, .two_op = 1 },
-    [0xf0] = {},
+    [0xf0] = { .two_op = 1 /* Mov */ },
 };
 
 static const opcode_desc_t xop_table[] = {
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -7693,11 +7693,6 @@ x86_emulate(
     case X86EMUL_OPC_VEX_F2(0x0f3a, 0xf0): /* rorx imm,r/m,r */
         vcpu_must_have(bmi2);
         generate_exception_if(vex.l || vex.reg != 0xf, X86_EXC_UD);
-        if ( ea.type == OP_REG )
-            src.val = *ea.reg;
-        else if ( (rc = read_ulong(ea.mem.seg, ea.mem.off, &src.val, op_bytes,
-                                   ctxt, ops)) != X86EMUL_OKAY )
-            goto done;
         if ( mode_64bit() && vex.w )
             asm ( "rorq %b1,%0" : "=g" (dst.val) : "c" (imm1), "0" (src.val) );
         else