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Subject: Re: [PATCH net-next 0/3] support changing steering policies in tuntap
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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From: Jason Wang <jasowang@redhat.com>
Message-ID: <fa7740de-9d7f-04ab-1b62-9ba52c5707a2@redhat.com>
Date: Fri, 29 Sep 2017 17:41:40 +0800
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On 2017年09月29日 00:09, Willem de Bruijn wrote:
> On Thu, Sep 28, 2017 at 3:23 AM, Jason Wang <jasowang@redhat.com> wrote:
>>
>> On 2017年09月28日 07:25, Willem de Bruijn wrote:
>>>>> In the future, both simple and sophisticated policy like RSS or other
>>>>> guest
>>>>> driven steering policies could be done on top.
>>>> IMHO there should be a more practical example before adding all this
>>>> indirection. And it would be nice to understand why this queue selection
>>>> needs to be tun specific.
>>> I was thinking the same and this reminds me of the various strategies
>>> implemented in packet fanout. tun_cpu_select_queue is analogous to
>>> fanout_demux_cpu though it is tun-specific in that it requires
>>> tun->numqueues.
>>
>> Right, the main idea is to introduce a way to change flow steering policy
>> for tun. I think fanout policy could be implemented through the API
>> introduced in this series. (Current flow caches based automatic steering
>> method is tun specific).
>>
>>> Fanout accrued various strategies until it gained an eBPF variant. Just
>>> supporting BPF is probably sufficient here, too.
>>
>> Technically yes, but for tun, it also serve for virt. We probably still need
>> some hard coded policy which could be changed by guest until we can accept
>> an BPF program from guest I think?
> When would a guest choose the policy? As long as this is under control
> of a host user, possibly unprivileged, allowing BPF here is moot, as any
> user can run socket filter BPF already. Programming from the guest is
> indeed different. I don't fully understand that use case.

The problem is userspace (qemu) know little about what kind of workloads 
will be done by guest, so we need guest controllable method here since 
it knows the best steering policy. Rethink about this, instead of 
passing eBPF from guest, qemu can have some pre-defined sets of polices. 
I will change the cpu id based to eBPF based in V2.

Thanks