Re: [PATCH 1/6] perf, core: Add generic transaction flags v4

[Peter Zijlstra]

On Fri, Sep 13, 2013 at 11:08:31AM -0700, Andi Kleen wrote:
>  /*
> + * Values for the transaction event qualifier, mostly for abort events.
> + * Multiple bits can be set.
> + */
> +enum {
> +	PERF_SAMPLE_TXN_ELISION        = (1 << 0), /* From elision */
> +	PERF_SAMPLE_TXN_TRANSACTION    = (1 << 1), /* From transaction */
> +	PERF_SAMPLE_TXN_SYNC           = (1 << 2), /* Instruction is related */
> +	PERF_SAMPLE_TXN_ASYNC          = (1 << 3), /* Instruction not related */
> +	PERF_SAMPLE_TXN_RETRY          = (1 << 4), /* Retry possible */
> +	PERF_SAMPLE_TXN_CONFLICT       = (1 << 5), /* Conflict abort */
> +	PERF_SAMPLE_TXN_CAPACITY_WRITE = (1 << 6), /* Capacity write abort */
> +	PERF_SAMPLE_TXN_CAPACITY_READ  = (1 << 7), /* Capacity read abort */
> +
> +	PERF_SAMPLE_TXN_MAX	       = (1 << 8),  /* non-ABI */
> +
> +	/* bits 24..31 are reserved for the abort code */
> +
> +	PERF_SAMPLE_TXN_ABORT_MASK  = 0xff000000,
> +	PERF_SAMPLE_TXN_ABORT_SHIFT = 24,
> +};

Why bits 24..31 ? Why not push the abort code into the upper 32 bits?
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