Re: [PATCH bpf-next v4 07/17] libbpf: Support drivers with non-combined channels

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Wed, 12 Jun 2019 15:56:48 +0000, Maxim Mikityanskiy wrote:
> Currently, libbpf uses the number of combined channels as the maximum
> queue number. However, the kernel has a different limitation:
> 
> - xdp_reg_umem_at_qid() allows up to max(RX queues, TX queues).
> 
> - ethtool_set_channels() checks for UMEMs in queues up to
>   combined_count + max(rx_count, tx_count).
> 
> libbpf shouldn't limit applications to a lower max queue number. Account
> for non-combined RX and TX channels when calculating the max queue
> number. Use the same formula that is used in ethtool.
> 
> Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
> Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
> Acked-by: Saeed Mahameed <saeedm@mellanox.com>

I don't think this is correct.  max_tx tells you how many TX channels
there can be, you can't add that to combined.  Correct calculations is:

max_num_chans = max(max_combined, max(max_rx, max_tx))

>  tools/lib/bpf/xsk.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index bf15a80a37c2..86107857e1f0 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -334,13 +334,13 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
>  		goto out;
>  	}
>  
> -	if (channels.max_combined == 0 || errno == EOPNOTSUPP)
> +	ret = channels.max_combined + max(channels.max_rx, channels.max_tx);
> +
> +	if (ret == 0 || errno == EOPNOTSUPP)
>  		/* If the device says it has no channels, then all traffic
>  		 * is sent to a single stream, so max queues = 1.
>  		 */
>  		ret = 1;
> -	else
> -		ret = channels.max_combined;
>  
>  out:
>  	close(fd);