netdev - Re: [PATCH bpf-next 2/6] bpf, net: rework cookie generator as per-cpu one

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Message-ID: <20200925083157.21df654d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 25 Sep 2020 08:31:57 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
        john.fastabend@...il.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 2/6] bpf, net: rework cookie generator as
 per-cpu one

On Fri, 25 Sep 2020 17:15:17 +0200 Eric Dumazet wrote:
> On 9/25/20 5:00 PM, Jakub Kicinski wrote:
> > Is this_cpu_inc() in itself atomic?

To answer my own question - it is :)

> >                     unlikely((val & (COOKIE_LOCAL_BATCH - 1)) == 0))
> > 
> > Can we reasonably assume we won't have more than 4k CPUs and just
> > statically divide this space by encoding CPU id in top bits?  
> 
> This might give some food to side channel attacks, since this would
> give an indication of cpu that allocated the id.
> 
> Also, I hear that some distros enabled 8K cpus.

Ok :(