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

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Date:   Fri, 25 Sep 2020 17:45:35 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        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 9/25/20 5:31 PM, Jakub Kicinski wrote:
> 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 :(
> 

I was not really serious about the side channel attacks, just some
thought about possible implications :)

Even with 8192 max cpus, splitting space into 2^(64-13) blocks would be fine I think.