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

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Message-ID: <83a0b3e4-1327-c1c4-4eb4-9a25ff533d1d@redhat.com>
Date:   Fri, 25 Nov 2022 16:29:29 +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 11/23/22 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?

> 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
-- 
Adrián Moreno

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