netdev - Re: [PATCH v3 2/2] net: core: support XDP generic on stacked devices.

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Date:   Fri, 24 May 2019 12:07:15 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     brouer@...hat.com, Saeed Mahameed <saeedm@...lanox.com>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "sthemmin@...rosoft.com" <sthemmin@...rosoft.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] net: core: support XDP generic on stacked
 devices.

On Fri, 24 May 2019 12:17:27 +0800
Jason Wang <jasowang@...hat.com> wrote:

> > Maybe this is acceptable, but it should be documented, as the current
> > assumption dictates: XDP program runs on the core where the XDP
> > frame/SKB was first seen.  
> 
> 
> At lest for TUN, this is not true. XDP frames were built by vhost_net 
> and passed to TUN. There's no guarantee that vhost_net kthread won't 
> move to another core.

This sound a little scary, as we depend on per-CPU variables (e.g.
bpf_redirect_info).  Can the vhost_net kthread move between CPUs
within/during the NAPI-poll?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer