netdev - Re: 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:   Sat, 9 Feb 2019 00:18:31 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "brouer@...hat.com" <brouer@...hat.com>
CC:     "thoiland@...hat.com" <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>,
        "toke@...e.dk" <toke@...e.dk>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "dsahern@...il.com" <dsahern@...il.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>
Subject: Re: Resource management for ndo_xdp_xmit (Was: [PATCH net]
 virtio_net: Account for tx bytes and packets on sending xdp_frames)

On Fri, 2019-02-08 at 15:17 -0800, Saeed Mahameed wrote:
> On Thu, 2019-02-07 at 19:08 +0000, Saeed Mahameed wrote:
> > 
> > So 
> > 1) on dev_map_update_elem() we will call
> > dev->dev->ndo_bpf() to notify the device on the intention to
> > start/stop
> > redirect, and wait for it to create/destroy the HW resources
> > before/after actually updating the map
> > 
> 
> silly me, dev_map_update_elem must be atomic, we can't hook driver
> resource allocation to it, it must come as a separate request
> (syscall)
> from user space to request to create XDP redirect resources.
> 

Well, it is possible to render dev_map_update_elem non-atomic and fail
BPF programs who try to update it in the verifier
check_map_func_compatibility.

if you know of any case where devmap needs to be updated from the BPF
program please let me know.