netdev - Resource management for ndo_xdp_xmit (Was: [PATCH net] virtio_net: Account for tx bytes and packets on sending xdp

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Date:   Thu, 7 Feb 2019 08:48:15 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     "dsahern@...il.com" <dsahern@...il.com>,
        Toke Høiland-Jørgensen 
        <thoiland@...hat.com>, "hawk@...nel.org" <hawk@...nel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "borkmann@...earbox.net" <borkmann@...earbox.net>,
        Tariq Toukan <tariqt@...lanox.com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "makita.toshiaki@....ntt.co.jp" <makita.toshiaki@....ntt.co.jp>,
        brouer@...hat.com,
        Toke Høiland-Jørgensen <toke@...e.dk>
Subject: Resource management for ndo_xdp_xmit (Was: [PATCH net] virtio_net:
 Account for tx bytes and packets on sending xdp_frames)


On Wed, 6 Feb 2019 00:06:33 +0000 Saeed Mahameed <saeedm@...lanox.com> wrote:

> On Mon, 2019-02-04 at 19:13 -0800, David Ahern wrote:
[...]
> > 
> > mlx5 needs some work. As I recall it still has the bug/panic
> > removing xdp programs - at least I don't recall seeing a patch for
> > it.  
> 
> Only when xdp_redirect to mlx5, and removing the program while
> redirect is happening, this is actually due to a lack of
> synchronization means between different drivers, we have some ideas
> to overcome this using a standard XDP API, or just use a hack in mlx5
> driver which i don't like:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/saeed/linux.git/commit/?h=topic/xdp-redirect-fix&id=a3652d03cc35fd3ad62744986c8ccaca74c9f20c
> 
> I will be working on this towards the end of this week.

Toke and I have been discussing how to solve this.

The main idea for fixing this is to tie resource allocation to interface
insertion into interface maps (kernel/bpf/devmap.c). As the =devmap=
already have the needed synchronisation mechanisms and steps for safely
adding and removing =net_devices= (e.g. stopping RX side, flushing
remaining frames, waiting RCU period before freeing objects, etc.)

As described here:
 https://github.com/xdp-project/xdp-project/blob/master/xdp-project.org#better-ndo_xdp_xmit-resource-management

--Jesper