Re: [Xen-devel] [PATCH v3 2/3] x86/emulate: add support of emulating SSE2 instruction {, v}movd mm, r32/m32 and {, v}movq mm, r64

[Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>]

On Monday 01 August 2016 06:59:08 Jan Beulich wrote:
> >>> On 01.08.16 at 14:53, <mdontu@xxxxxxxxxxxxxxx> wrote:  
> > On Monday 01 August 2016 10:52:12 Andrew Cooper wrote:  
> >> On 01/08/16 03:52, Mihai Donțu wrote:  
> >> > Found that Windows driver was using a SSE2 instruction MOVD.
> >> >
> >> > Signed-off-by: Zhi Wang <zhi.a.wang@xxxxxxxxx>
> >> > Signed-off-by: Mihai Donțu <mdontu@xxxxxxxxxxxxxxx>
> >> > ---
> >> > Picked from the XenServer 7 patch queue, as suggested by Andrew Cooper
> >> >
> >> > Changed since v2:
> >> >  * handle the case where the destination is a GPR
> >> > ---
> >> >  xen/arch/x86/x86_emulate/x86_emulate.c | 38   
> > +++++++++++++++++++++++++++++++---  
> >> >  1 file changed, 35 insertions(+), 3 deletions(-)
> >> >
> >> > diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c   
> > b/xen/arch/x86/x86_emulate/x86_emulate.c  
> >> > index 44de3b6..9f89ada 100644
> >> > --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> >> > +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> >> > @@ -204,7 +204,7 @@ static uint8_t twobyte_table[256] = {
> >> >      /* 0x60 - 0x6F */
> >> >      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ImplicitOps|ModRM,
> >> >      /* 0x70 - 0x7F */
> >> > -    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, 0, 0, 0, 0, 0, ImplicitOps|ModRM,   
> > ImplicitOps|ModRM,  
> >> >      /* 0x80 - 0x87 */
> >> >      ImplicitOps, ImplicitOps, ImplicitOps, ImplicitOps,
> >> >      ImplicitOps, ImplicitOps, ImplicitOps, ImplicitOps,
> >> > @@ -4409,6 +4409,10 @@ x86_emulate(
> >> >      case 0x6f: /* movq mm/m64,mm */
> >> >                 /* {,v}movdq{a,u} xmm/m128,xmm */
> >> >                 /* vmovdq{a,u} ymm/m256,ymm */
> >> > +    case 0x7e: /* movd mm,r/m32 */
> >> > +               /* movq mm,r/m64 */
> >> > +               /* {,v}movd xmm,r/m32 */
> >> > +               /* {,v}movq xmm,r/m64 */
> >> >      case 0x7f: /* movq mm,mm/m64 */
> >> >                 /* {,v}movdq{a,u} xmm,xmm/m128 */
> >> >                 /* vmovdq{a,u} ymm,ymm/m256 */
> >> > @@ -4432,7 +4436,17 @@ x86_emulate(
> >> >                  host_and_vcpu_must_have(sse2);
> >> >                  buf[0] = 0x66; /* SSE */
> >> >                  get_fpu(X86EMUL_FPU_xmm, &fic);
> >> > -                ea.bytes = (b == 0xd6 ? 8 : 16);
> >> > +                switch ( b )
> >> > +                {
> >> > +                case 0x7e:
> >> > +                    ea.bytes = 4;
> >> > +                    break;
> >> > +                case 0xd6:
> >> > +                    ea.bytes = 8;
> >> > +                    break;
> >> > +                default:
> >> > +                    ea.bytes = 16;
> >> > +                }
> >> >                  break;
> >> >              case vex_none:
> >> >                  if ( b != 0xe7 )
> >> > @@ -4452,7 +4466,17 @@ x86_emulate(
> >> >                      ((vex.pfx != vex_66) && (vex.pfx != vex_f3)));
> >> >              host_and_vcpu_must_have(avx);
> >> >              get_fpu(X86EMUL_FPU_ymm, &fic);
> >> > -            ea.bytes = (b == 0xd6 ? 8 : (16 << vex.l));
> >> > +            switch ( b )
> >> > +            {
> >> > +            case 0x7e:
> >> > +                ea.bytes = 4;
> >> > +                break;
> >> > +            case 0xd6:
> >> > +                ea.bytes = 8;
> >> > +                break;
> >> > +            default:
> >> > +                ea.bytes = 16 << vex.l;
> >> > +            }
> >> >          }
> >> >          if ( ea.type == OP_MEM )
> >> >          {
> >> > @@ -4468,6 +4492,14 @@ x86_emulate(
> >> >              vex.b = 1;
> >> >              buf[4] &= 0x38;
> >> >          }
> >> > +        else if ( b == 0x7e )
> >> > +        {
> >> > +            /* convert the GPR destination to (%rAX) */
> >> > +            *((unsigned long *)&mmvalp) = (unsigned long)ea.reg;
> >> > +            rex_prefix &= ~REX_B;
> >> > +            vex.b = 1;
> >> > +            buf[4] &= 0x38;
> >> > +        }    
> >> 
> >> Thankyou for doing this.  However, looking at it, it has some code in
> >> common with the "ea.type == OP_MEM" clause.
> >> 
> >> Would this work?
> >> 
> >> diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c
> >> b/xen/arch/x86/x86_emulate/x86_emulate.c
> >> index fe594ba..90db067 100644
> >> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> >> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> >> @@ -4453,16 +4453,25 @@ x86_emulate(
> >>              get_fpu(X86EMUL_FPU_ymm, &fic);
> >>              ea.bytes = 16 << vex.l;
> >>          }
> >> -        if ( ea.type == OP_MEM )
> >> +        if ( ea.type == OP_MEM || ea.type == OP_REG )
> >>          {
> >> -            /* XXX enable once there is ops->ea() or equivalent
> >> -            generate_exception_if((vex.pfx == vex_66) &&
> >> -                                  (ops->ea(ea.mem.seg, ea.mem.off)
> >> -                                   & (ea.bytes - 1)), EXC_GP, 0); */
> >> -            if ( b == 0x6f )
> >> -                rc = ops->read(ea.mem.seg, ea.mem.off+0, mmvalp,
> >> -                               ea.bytes, ctxt);
> >>              /* convert memory operand to (%rAX) */
> >> +
> >> +            if ( ea.type == OP_MEM)
> >> +            {
> >> +                /* XXX enable once there is ops->ea() or equivalent
> >> +                   generate_exception_if((vex.pfx == vex_66) &&
> >> +                   (ops->ea(ea.mem.seg, ea.mem.off)
> >> +                   & (ea.bytes - 1)), EXC_GP, 0); */
> >> +                if ( b == 0x6f )
> >> +                    rc = ops->read(ea.mem.seg, ea.mem.off+0, mmvalp,
> >> +                                   ea.bytes, ctxt);
> >> +            }
> >> +            else if ( ea.type == OP_REG )
> >> +            {
> >> +                *((unsigned long *)&mmvalp) = (unsigned long)ea.reg;
> >> +            }
> >> +
> >>              rex_prefix &= ~REX_B;
> >>              vex.b = 1;
> >>              buf[4] &= 0x38;
> >> 
> >> 
> >> This is untested, but avoids duplicating this bit of state maniupulation.  
> > 
> > Your suggestion makes sense, but I'm starting to doubt my initial
> > patch. :-) I'm testing "movq xmm1, xmm1" and noticing that it takes the
> > GPR-handling route and I can't seem to be able to easily prevent it
> > with !(rex_prefix & REX_B), as rex_prefix == 0 and vex.b == 1. I need
> > to take a harder look at how that class of instructions is coded.  
> 
> You obviously need to distinguish the two kinds of register sources/
> destinations: GPRs need suitable re-writing of the instruction (without
> having looked at the most recent version of the patch yet I btw doubt
> converting register to memory operands is the most efficient model),
> while MMs, XMMs, and YMMs can retain their register encoding.

Regarding efficiency, I'm not married with the approach I've proposed.
If you can give me a few more hints, I can give it a try.

-- 
Mihai DONȚU

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