netdev - Re: [ovs-dev] [RFC net-next 1/6] openvswitch: exclude kernel flow key from upcalls

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Date:   Mon, 28 Nov 2022 10:12:21 +0100
From:   Adrian Moreno <amorenoz@...hat.com>
To:     Ilya Maximets <i.maximets@....org>,
        Aaron Conole <aconole@...hat.com>, netdev@...r.kernel.org
Cc:     dev@...nvswitch.org, linux-kernel@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        linux-kselftest@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Shuah Khan <shuah@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [ovs-dev] [RFC net-next 1/6] openvswitch: exclude kernel flow key
 from upcalls

On 11/25/22 16:51, Ilya Maximets wrote:
> On 11/25/22 16:29, Adrian Moreno wrote:
>>
>>
>> On 11/23/22 22:22, Ilya Maximets wrote:
>>> On 11/22/22 15:03, Aaron Conole wrote:
>>>> When processing upcall commands, two groups of data are available to
>>>> userspace for processing: the actual packet data and the kernel
>>>> sw flow key data.  The inclusion of the flow key allows the userspace
>>>> avoid running through the dissection again.
>>>>
>>>> However, the userspace can choose to ignore the flow key data, as is
>>>> the case in some ovs-vswitchd upcall processing.  For these messages,
>>>> having the flow key data merely adds additional data to the upcall
>>>> pipeline without any actual gain.  Userspace simply throws the data
>>>> away anyway.
>>>
>>> Hi, Aaron.  While it's true that OVS in userpsace is re-parsing the
>>> packet from scratch and using the newly parsed key for the OpenFlow
>>> translation, the kernel-porvided key is still used in a few important
>>> places.  Mainly for the compatibility checking.  The use is described
>>> here in more details:
>>>     https://docs.kernel.org/networking/openvswitch.html#flow-key-compatibility
>>>
>>> We need to compare the key generated in userspace with the key
>>> generated by the kernel to know if it's safe to install the new flow
>>> to the kernel, i.e. if the kernel and OVS userpsace are parsing the
>>> packet in the same way.
>>>
>>
>> Hi Ilya,
>>
>> Do we need to do that for every packet?
>> Could we send a bitmask of supported fields to userspace at feature
>> negotiation and let OVS slowpath flows that it knows the kernel won't
>> be able to handle properly?
> 
> It's not that simple, because supported fields in a packet depend
> on previous fields in that same packet.  For example, parsing TCP
> header is generally supported, but it won't be parsed for IPv6
> fragments (even the first one), number of vlan headers will affect
> the parsing as we do not parse deeper than 2 vlan headers, etc.
> So, I'm afraid we have to have a per-packet information, unless we
> can somehow probe all the possible valid combinations of packet
> headers.
> 

Surely. I understand that we'd need more than just a bit per field. Things like 
L4 on IPv6 frags would need another bit and the number of VLAN headers would 
need some more. But, are these a handful of exceptions or do we really need all 
the possible combinations of headers? If it's a matter of naming a handful of 
corner cases I think we could consider expressing them at initialization time 
and safe some buffer space plus computation time both in kernel and userspace.

-- 
Adrián Moreno

>>
>>
>>> On the other hand, OVS today doesn't check the data, it only checks
>>> which fields are present.  So, if we can generate and pass the bitmap
>>> of fields present in the key or something similar without sending the
>>> full key, that might still save some CPU cycles and memory in the
>>> socket buffer while preserving the ability to check for forward and
>>> backward compatibility.  What do you think?
>>>
>>>
>>> The rest of the patch set seems useful even without patch #1 though.
>>>
>>> Nit: This patch #1 should probably be merged with the patch #6 and be
>>> at the end of a patch set, so the selftest and the main code are updated
>>> at the same time.
>>>
>>> Best regards, Ilya Maximets.
>>> _______________________________________________
>>> dev mailing list
>>> dev@...nvswitch.org
>>> https://mail.openvswitch.org/mailman/listinfo/ovs-dev
>>>
>>
>> Thanks
> 

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