Re: [Qemu-devel] [RFC PATCH 2/3] tcg: Add support for fence generation in x86 backend

[Richard Henderson <rth@twiddle.net>]

On 05/24/2016 10:18 AM, Pranith Kumar wrote:
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
>  tcg/i386/tcg-target.h     | 1 +
>  tcg/i386/tcg-target.inc.c | 9 +++++++++
>  tcg/tcg-opc.h             | 2 +-
>  tcg/tcg.c                 | 1 +
>  4 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/tcg/i386/tcg-target.h b/tcg/i386/tcg-target.h
> index 92be341..93ea42e 100644
> --- a/tcg/i386/tcg-target.h
> +++ b/tcg/i386/tcg-target.h
> @@ -100,6 +100,7 @@ extern bool have_bmi1;
>  #define TCG_TARGET_HAS_muls2_i32        1
>  #define TCG_TARGET_HAS_muluh_i32        0
>  #define TCG_TARGET_HAS_mulsh_i32        0
> +#define TCG_TARGET_HAS_fence            1

This has to be defined for all hosts.

The default implementation should be a function call into tcg-runtime.c that 
calls smp_mb().

> @@ -347,6 +347,7 @@ static inline int tcg_target_const_match(tcg_target_long val, TCGType type,
>  #define OPC_SHRX        (0xf7 | P_EXT38 | P_SIMDF2)
>  #define OPC_TESTL	(0x85)
>  #define OPC_XCHG_ax_r32	(0x90)
> +#define OPC_MFENCE      (0xAE | P_EXT)
>
>  #define OPC_GRP3_Ev	(0xf7)
>  #define OPC_GRP5	(0xff)
> @@ -686,6 +687,14 @@ static inline void tcg_out_pushi(TCGContext *s, tcg_target_long val)
>      }
>  }
>
> +static inline void tcg_out_fence(TCGContext *s)
> +{
> +    /* TODO: Figure out an appropriate place for the encoding */
> +    tcg_out8(s, 0x0F);
> +    tcg_out8(s, 0xAE);
> +    tcg_out8(s, 0xF0);
> +}

Why define OPC_MFENCE if you're not going to use it?  Of course, it's not 
exactly a complete and useful definition, so maybe just delete OPC_MFENCE.

Also, for 32-bit you need to check for sse2 before outputting this.  See also 
the existing cpuid checks in tcg_target_init and the fallback smp_mb definition 
for pre-gcc-4.4.


r~