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From: Richard Henderson <rth@twiddle.net>
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Subject: Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for
 memory barrier
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To: Pranith Kumar <bobby.prani@gmail.com>, "open list:All patches CC here" <qemu-devel@nongnu.org>
Cc: alex.bennee@linaro.org, serge.fdrv@linaro.org

On 05/31/2016 11:39 AM, Pranith Kumar wrote:
> +********* Memory Barrier support
> +
> +* mb <$arg>

Document what $arg should be.

> +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

The argument is *not* optional.

> +void tcg_gen_mb(TCGArg a)
> +{
> +    /* ??? Enable only when MTTCG is enabled.  */
> +    tcg_gen_op1(&tcg_ctx, INDEX_op_mb, 0);

Pass A to tcg_gen_op1, not 0.

> +/* TCGOpmb args */
> +#define TCG_MB_FULL             ((TCGArg)(0))
> +#define TCG_MB_READ             ((TCGArg)(1))
> +#define TCG_MB_WRITE            ((TCGArg)(2))
> +#define TCG_MB_ACQUIRE          ((TCGArg)(3))
> +#define TCG_MB_RELEASE          ((TCGArg)(4))

This is, IMO, confused.  Either we should use the C++11 barrier types, or the 
Linux barrier types, but not both.


r~