Re: [PATCH v4 04/17] x86emul: support {, V}{, U}COMIS{S, D}

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 28/02/17 12:51, Jan Beulich wrote:
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> v4: Add missing copy_REX_VEX().
> v3: Ignore VEX.l. Add fic.exn_raised constraint to invoke_stub() use.
> v2: Add missing RET to stub. Generate #UD (instead of simply failing)
>     when VEX.l is disallowed.
>
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -254,7 +254,7 @@ static const struct {
>      [0x2a] = { DstImplicit|SrcMem|ModRM|Mov, simd_other },
>      [0x2b] = { DstMem|SrcImplicit|ModRM|Mov, simd_any_fp },
>      [0x2c ... 0x2d] = { DstImplicit|SrcMem|ModRM|Mov, simd_other },
> -    [0x2e ... 0x2f] = { ImplicitOps|ModRM },
> +    [0x2e ... 0x2f] = { ImplicitOps|ModRM|TwoOp },
>      [0x30 ... 0x35] = { ImplicitOps },
>      [0x37] = { ImplicitOps },
>      [0x38] = { DstReg|SrcMem|ModRM },
> @@ -5468,6 +5468,55 @@ x86_emulate(
>          state->simd_size = simd_none;
>          break;
>  
> +    CASE_SIMD_PACKED_FP(, 0x0f, 0x2e):     /* ucomis{s,d} xmm/mem,xmm */
> +    CASE_SIMD_PACKED_FP(_VEX, 0x0f, 0x2e): /* vucomis{s,d} xmm/mem,xmm */
> +    CASE_SIMD_PACKED_FP(, 0x0f, 0x2f):     /* comis{s,d} xmm/mem,xmm */
> +    CASE_SIMD_PACKED_FP(_VEX, 0x0f, 0x2f): /* vcomis{s,d} xmm/mem,xmm */
> +        if ( vex.opcx == vex_none )
> +        {
> +            if ( vex.pfx )
> +                vcpu_must_have(sse2);
> +            else
> +                vcpu_must_have(sse);
> +            get_fpu(X86EMUL_FPU_xmm, &fic);
> +        }
> +        else
> +        {
> +            host_and_vcpu_must_have(avx);
> +            get_fpu(X86EMUL_FPU_ymm, &fic);
> +        }

This is starting to become a common sequence.  Is there any sensible way
to factor it out in a non-macro way, to avoid the compiler instantiating
it at the top of many basic blocks?

> +
> +        opc = init_prefixes(stub);
> +        opc[0] = b;
> +        opc[1] = modrm;
> +        if ( ea.type == OP_MEM )
> +        {
> +            rc = ops->read(ea.mem.seg, ea.mem.off, mmvalp, vex.pfx ? 8 : 4,
> +                           ctxt);
> +            if ( rc != X86EMUL_OKAY )
> +                goto done;
> +
> +            /* Convert memory operand to (%rAX). */
> +            rex_prefix &= ~REX_B;
> +            vex.b = 1;
> +            opc[1] &= 0x38;
> +        }
> +        fic.insn_bytes = PFX_BYTES + 2;
> +        opc[2] = 0xc3;
> +
> +        copy_REX_VEX(opc, rex_prefix, vex);
> +        invoke_stub(_PRE_EFLAGS("[eflags]", "[mask]", "[tmp]"),
> +                    _POST_EFLAGS("[eflags]", "[mask]", "[tmp]"),
> +                    [eflags] "+g" (_regs._eflags),
> +                    [tmp] "=&r" (cr4 /* dummy */), "+m" (*mmvalp),

This is latently dangerous.  It would be better to have an explicit
"unsigned long dummy;", which the compiler will perfectly easily elide
during register scheduling.

~Andrew

> +                    "+m" (fic.exn_raised)
> +                    : [func] "rm" (stub.func), "a" (mmvalp),
> +                      [mask] "i" (EFLAGS_MASK));
> +
> +        put_stub(stub);
> +        put_fpu(&fic);
> +        break;
> +
>      case X86EMUL_OPC(0x0f, 0x30): /* wrmsr */
>          generate_exception_if(!mode_ring0(), EXC_GP, 0);
>          fail_if(ops->write_msr == NULL);
>
>
>


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