netdev - Re: [PATCH net-next 0/2] Introduce XDP_FLAGS_NO

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Message-ID: <ee57b0ed-89e2-675e-b080-0059c181a2be@redhat.com>
Date:   Tue, 12 Jan 2021 11:03:53 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Charlie Somerville <charlie@...rlie.bz>, davem@...emloft.net,
        kuba@...nel.org, mst@...hat.com
Cc:     netdev@...r.kernel.org, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH net-next 0/2] Introduce XDP_FLAGS_NO_TX flag


On 2021/1/9 上午10:49, Charlie Somerville wrote:
> This patch series introduces a new flag XDP_FLAGS_NO_TX which prevents
> the allocation of additional send queues for XDP programs.


This part I don't understand. Is such flag a must? I think the answer is 
probably not.

Why not simply do:

1) if we had sufficient TX queues, use dedicated TX queues for XDP_TX
2) if we don't, simple synchronize through spin_lock[1]

Thanks

[1] https://www.spinics.net/lists/bpf/msg32587.html


>
> Included in this patch series is an implementation of XDP_FLAGS_NO_TX
> for the virtio_net driver. This flag is intended to be advisory - not
> all drivers must implement support for it.
>
> Many virtualised environments only provide enough virtio_net send queues
> for the number of processors allocated to the VM:
>
> # nproc
> 8
> # ethtool --show-channels ens3
> Channel parameters for ens3:
> Pre-set maximums:
> RX:     0
> TX:     0
> Other:      0
> Combined:   8
>
> In this configuration XDP is unusable because the virtio_net driver
> always tries to allocate an extra send queue for each processor - even
> if the XDP the program never uses the XDP_TX functionality.
>
> While XDP_TX is still unavailable in these environments, this new flag
> expands the set of XDP programs that can be used.
>
> This is my first contribution to the kernel, so apologies if I've sent
> this to the wrong list. I have tried to cc relevant maintainers but
> it's possible I may have missed some people. I'm looking forward to
> receiving feedback on this change.
>
> Charlie Somerville (2):
>    xdp: Add XDP_FLAGS_NO_TX flag
>    virtio_net: Implement XDP_FLAGS_NO_TX support
>
>   drivers/net/virtio_net.c     | 17 +++++++++++++----
>   include/uapi/linux/if_link.h |  5 ++++-
>   2 files changed, 17 insertions(+), 5 deletions(-)
>