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Date:   Fri, 8 Feb 2019 23:17:58 +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 Thu, 2019-02-07 at 19:08 +0000, Saeed Mahameed wrote:
> On Thu, 2019-02-07 at 08:48 +0100, Jesper Dangaard Brouer wrote:
> > 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
> 
> Yes you already suggested this approach @LPC:
> 
> 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.


> But:
> 2) this won't totally solve our problem, since sometimes the driver
> can
> decide to recreate (change of configuration) hw resources on the fly
> while redirect/devmap is already happening, so we need some kind of a
> dev_map_notification or a flag with rcu synch, for when the driver
> want
> to make the xdp redirect resources unavailable.
> 

I will focus on this problem first, then figure out how to create XDP
redirect resources without actullay attaching a dummy xdp program.

> Thanks,
> Saeed.

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