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

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Date:   Thu, 13 Jun 2019 14:41:30 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Maxim Mikityanskiy <maximmi@...lanox.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jonathan Lemon <bsd@...com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
Subject: Re: [PATCH bpf-next v4 07/17] libbpf: Support drivers with
 non-combined channels

On Wed, 12 Jun 2019 at 22:24, Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> 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@...lanox.com>
> > Reviewed-by: Tariq Toukan <tariqt@...lanox.com>
> > Acked-by: Saeed Mahameed <saeedm@...lanox.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))
>

...but the inner max should be min, right?

Assuming we'd like to receive and send.