Re: [Xen-devel] [PATCH v3 1/3] x86/emulate: add support for {, v}movq xmm, xmm/m64

[Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>]

On Monday 01 August 2016 07:19:51 Jan Beulich wrote:
> >>> On 01.08.16 at 04:52, <mdontu@xxxxxxxxxxxxxxx> wrote:  
> > @@ -4412,6 +4412,7 @@ x86_emulate(
> >      case 0x7f: /* movq mm,mm/m64 */
> >                 /* {,v}movdq{a,u} xmm,xmm/m128 */
> >                 /* vmovdq{a,u} ymm,ymm/m256 */
> > +    case 0xd6: /* {,v}movq xmm,xmm/m64 */
> >      {
> >          uint8_t *buf = get_stub(stub);
> >          struct fpu_insn_ctxt fic = { .insn_bytes = 5 };
> > @@ -4429,9 +4430,9 @@ x86_emulate(
> >              case vex_66:
> >              case vex_f3:
> >                  host_and_vcpu_must_have(sse2);
> > -                buf[0] = 0x66; /* movdqa */
> > +                buf[0] = 0x66; /* SSE */  
> 
> The comment change here indicates a problem: So far it was indicating
> that despite the possible F3 prefix (movdqu) we encode a 66 one
> (movdqa). Opcode D6 prefixed with F3, however, is movq2dq, which
> you then either don't emulate correctly, or if it happens to be
> emulated correctly you should include in the comment accompanying
> the case label. And its AVX counterpart should then produce #UD.

I fiddled with this for a while and the attached patch (adjusted)
appears to be doing the right thing: ie. movq2dq gets emulated
correctly too. copy_REX_VEX() does not work OK with movq2dq, but it
looked easy to single out this case.

All tests pass, including for {,v}movq xmm/m64 and movq2dq. There does
not appear to be an AVX variant for the latter, or I'm not reading the
Intel SDM right (or binutils' as is lying to me).

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index fe594ba..d6c199b 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -245,7 +245,7 @@ static uint8_t twobyte_table[256] = {
     ImplicitOps, ImplicitOps, ImplicitOps, ImplicitOps,
     ImplicitOps, ImplicitOps, ImplicitOps, ImplicitOps,
     /* 0xD0 - 0xDF */
-    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, ImplicitOps|ModRM, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     /* 0xE0 - 0xEF */
     0, 0, 0, 0, 0, 0, 0, ImplicitOps|ModRM, 0, 0, 0, 0, 0, 0, 0, 0,
     /* 0xF0 - 0xFF */
@@ -4412,6 +4412,8 @@ x86_emulate(
     case 0x7f: /* movq mm,mm/m64 */
                /* {,v}movdq{a,u} xmm,xmm/m128 */
                /* vmovdq{a,u} ymm,ymm/m256 */
+    case 0xd6: /* {,v}movq xmm,xmm/m64 */
+               /* movq2dq mm,xmm */
     {
         uint8_t *buf = get_stub(stub);
         struct fpu_insn_ctxt fic = { .insn_bytes = 5 };
@@ -4431,7 +4433,7 @@ x86_emulate(
                 host_and_vcpu_must_have(sse2);
                 buf[0] = 0x66; /* movdqa */
                 get_fpu(X86EMUL_FPU_xmm, &fic);
-                ea.bytes = 16;
+                ea.bytes = (b == 0xd6 ? 8 : 16);
                 break;
             case vex_none:
                 if ( b != 0xe7 )
@@ -4451,7 +4453,7 @@ x86_emulate(
                     ((vex.pfx != vex_66) && (vex.pfx != vex_f3)));
             host_and_vcpu_must_have(avx);
             get_fpu(X86EMUL_FPU_ymm, &fic);
-            ea.bytes = 16 << vex.l;
+            ea.bytes = (b == 0xd6 ? 8 : (16 << vex.l));
         }
         if ( ea.type == OP_MEM )
         {
@@ -4469,7 +4471,11 @@ x86_emulate(
         }
         if ( !rc )
         {
-           copy_REX_VEX(buf, rex_prefix, vex);
+           /* try to preserve the mandatory prefix for movq2dq */
+           if ( !rex_prefix && vex.opcx == vex_none && vex.pfx == vex_f3 )
+               buf[0] = 0xf3;
+           else
+               copy_REX_VEX(buf, rex_prefix, vex);
            asm volatile ( "call *%0" : : "r" (stub.func), "a" (mmvalp)
                                      : "memory" );
         }

-- 
Mihai DONȚU

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