Re: [PATCH 1/2] ixgbe: fix VF statistic wraparound handling macro

[Alexander Duyck <alexander.duyck@gmail.com>]

On 10/12/2015 06:33 AM, Harry van Haaren wrote:
> Fix a misinterpretation of VF stats in ixgbe
>
> Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
> ---
>   drivers/net/ixgbe/ixgbe_ethdev.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ixgbe/ixgbe_ethdev.c b/drivers/net/ixgbe/ixgbe_ethdev.c
> index ec2918c..d226e8d 100644
> --- a/drivers/net/ixgbe/ixgbe_ethdev.c
> +++ b/drivers/net/ixgbe/ixgbe_ethdev.c
> @@ -329,10 +329,14 @@ static int ixgbe_timesync_read_tx_timestamp(struct rte_eth_dev *dev,
>   /*
>    * Define VF Stats MACRO for Non "cleared on read" register
>    */
> -#define UPDATE_VF_STAT(reg, last, cur)	                        \
> +#define UPDATE_VF_STAT(reg, last, cur)                          \
>   {                                                               \
>   	uint32_t latest = IXGBE_READ_REG(hw, reg);              \
> -	cur += latest - last;                                   \
> +	if(likely(latest > last)) {                             \
> +		cur += latest - last;                           \
> +	} else {                                                \
> +		cur += (UINT_MAX - last) + latest;              \
> +	}                                                       \
>   	last = latest;                                          \
>   }
>   

 From what I can tell your math is adding an off by one error.  You 
should probably be using UINT_MAX as a mask for the result, not as a 
part of the calculation itself.

So the correct way to compute this would be "cur += (latest - last) & 
UINT_MAX".  Also the mask approach should be faster as it avoids any 
conditional jumps.

- Alex