Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier

[Sergey Fedorov]

On 31/05/16 21:39, Pranith Kumar wrote:
> diff --git a/tcg/README b/tcg/README
> index f4a8ac1..cfe79d7 100644
> --- a/tcg/README
> +++ b/tcg/README
> @@ -402,6 +402,23 @@ double-word product T0.  The later is returned in two 
> single-word outputs.
>  
>  Similar to mulu2, except the two inputs T1 and T2 are signed.
>  
> +********* Memory Barrier support
> +
> +* mb <$arg>
> +
> +Generate a target memory barrier instruction to ensure memory ordering as 
> being
> +enforced by a corresponding guest memory barrier instruction. The ordering
> +enforced by the backend may be stricter than the ordering required by the 
> guest.
> +It cannot be weaker. This opcode takes an optional constant argument if 
> required
> +to generate the appropriate barrier instruction. The backend should take 
> care to
> +emit the target barrier instruction only when necessary i.e., for SMP guests 
> and
> +when MTTCG is enabled.
> +
> +The guest translators should generate this opcode for all guest instructions
> +which have ordering side effects.

I think we need to extend TCG load/store instruction attributes to
provide information about guest ordering requirements and leave this TCG
operation only for explicit barrier instruction translation.

> +
> +Please see docs/atomics.txt for more information on memory barriers.
> +
>  ********* 64-bit guest on 32-bit host support
>  
>  The following opcodes are internal to TCG.  Thus they are to be implemented 
> by
> diff --git a/tcg/tcg-op.c b/tcg/tcg-op.c
> index f554b86..a6f01a7 100644
> --- a/tcg/tcg-op.c
> +++ b/tcg/tcg-op.c
> @@ -143,6 +143,12 @@ void tcg_gen_op6(TCGContext *ctx, TCGOpcode opc, TCGArg 
> a1, TCGArg a2,
>      tcg_emit_op(ctx, opc, pi);
>  }
>  
> +void tcg_gen_mb(TCGArg a)
> +{
> +    /* ??? Enable only when MTTCG is enabled.  */
> +    tcg_gen_op1(&tcg_ctx, INDEX_op_mb, 0);

Yes, this could be a right place to check for MTTCG enabled and number
of CPUs emulated. If we do it here, then we should adjust the
documentation stating that the backend should take care of it.

Kind regards,
Sergey