netdev - Re: [PATCH v2 bpf 1/5] net: ethtool: add xdp properties flag set

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Date:   Mon, 7 Dec 2020 18:01:00 -0700
From:   David Ahern <dsahern@...il.com>
To:     John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        alardam@...il.com, magnus.karlsson@...el.com,
        bjorn.topel@...el.com, andrii.nakryiko@...il.com, kuba@...nel.org,
        ast@...nel.org, netdev@...r.kernel.org, davem@...emloft.net,
        hawk@...nel.org, jonathan.lemon@...il.com, bpf@...r.kernel.org,
        jeffrey.t.kirsher@...el.com, maciejromanfijalkowski@...il.com,
        intel-wired-lan@...ts.osuosl.org,
        Marek Majtyka <marekx.majtyka@...el.com>
Subject: Re: [PATCH v2 bpf 1/5] net: ethtool: add xdp properties flag set

On 12/7/20 1:52 PM, John Fastabend wrote:
>>
>> I think we need to keep XDP_TX action separate, because I think that
>> there are use-cases where the we want to disable XDP_TX due to end-user
>> policy or hardware limitations.
> 
> How about we discover this at load time though. Meaning if the program
> doesn't use XDP_TX then the hardware can skip resource allocations for
> it. I think we could have verifier or extra pass discover the use of
> XDP_TX and then pass a bit down to driver to enable/disable TX caps.
> 

This was discussed in the context of virtio_net some months back - it is
hard to impossible to know a program will not return XDP_TX (e.g., value
comes from a map).

Flipping that around, what if the program attach indicates whether
XDP_TX could be returned. If so, driver manages the resource needs. If
not, no resource needed and if the program violates that and returns
XDP_TX the packet is dropped.