Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <83a0b3e4-1327-c1c4-4eb4-9a25ff533d1d@redhat.com>
Date:   Fri, 25 Nov 2022 16:29:29 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.5.0
Subject: Re: [ovs-dev] [RFC net-next 1/6] openvswitch: exclude kernel flow key
 from upcalls
Content-Language: en-US
To:     Ilya Maximets <i.maximets@ovn.org>,
        Aaron Conole <aconole@redhat.com>, netdev@vger.kernel.org
Cc:     dev@openvswitch.org, linux-kernel@vger.kernel.org,
        Eric Dumazet <edumazet@google.com>,
        linux-kselftest@vger.kernel.org, Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, Shuah Khan <shuah@kernel.org>,
        "David S. Miller" <davem@davemloft.net>
References: <20221122140307.705112-1-aconole@redhat.com>
 <20221122140307.705112-2-aconole@redhat.com>
 <c04242ee-f125-6d95-e263-65470222d3cf@ovn.org>
From:   Adrian Moreno <amorenoz@redhat.com>
In-Reply-To: <c04242ee-f125-6d95-e263-65470222d3cf@ovn.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk


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@openvswitch.org
> https://mail.openvswitch.org/mailman/listinfo/ovs-dev
> 

Thanks
-- 
Adrián Moreno