netdev - Re: [PATCH net-next 0/5] virtio-net: Add SCTP checksum offload support

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Date:   Wed, 18 Apr 2018 14:57:52 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Vlad Yasevich <vyasevic@...hat.com>,
        Vladislav Yasevich <vyasevich@...il.com>,
        network dev <netdev@...r.kernel.org>,
        linux-sctp@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Neil Horman <nhorman@...driver.com>
Subject: Re: [PATCH net-next 0/5] virtio-net: Add SCTP checksum offload support

On Wed, Apr 18, 2018 at 9:33 AM, Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
> On Tue, Apr 17, 2018 at 04:35:18PM -0400, Vlad Yasevich wrote:
>> On 04/02/2018 10:47 AM, Marcelo Ricardo Leitner wrote:
>> > On Mon, Apr 02, 2018 at 09:40:01AM -0400, Vladislav Yasevich wrote:
>> >> Now that we have SCTP offload capabilities in the kernel, we can add
>> >> them to virtio as well.  First step is SCTP checksum.
>> >
>> > Thanks.
>> >
>> >> As for GSO, the way sctp GSO is currently implemented buys us nothing
>> >> in added support to virtio.  To add true GSO, would require a lot of
>> >> re-work inside of SCTP and would require extensions to the virtio
>> >> net header to carry extra sctp data.
>> >
>> > Can you please elaborate more on this? Is this because SCTP GSO relies
>> > on the gso skb format for knowing how to segment it instead of having
>> > a list of sizes?
>> >
>>
>> it's mainly because all the true segmentation, placing data into chunks,
>> has already happened.  All that GSO does is allow for higher bundling
>> rate between VMs. If that is all SCTP GSO ever going to do, that fine,
>> but the goal is to do real GSO eventually and potentially reduce the
>> amount of memory copying we are doing.
The memory copying for bundling can't be avoided, as the chunks in
one packet may be from some different messages, as Marcelo said below.

>> If we do that, any current attempt at GSO in virtio would have to be
>> depricated and we'd need GSO2 or something like that.
Why would it be depreciated? virtio_net actually also supports frag_list,
all we need to do is to enable it.
I already have a patch for  sctp gso over virtio_net in my local tree:

patch on qemu-2.9.0(el7):
  https://paste.fedoraproject.org/paste/IgnSbW74L6P9aUZpnWHBsA
patch on kernel-4.16(linus):
  https://paste.fedoraproject.org/paste/L7AhFIVVp3k8VfEV29QMiw

The performance has been improved quite a lot with this:
testing over virtio-net(guest):
  https://paste.fedoraproject.org/paste/cxQMnrDQDf9AoNgBpA-twA
testing over tap(host):
  https://paste.fedoraproject.org/paste/QPdc22pgvF-x68oKDjWa9g

>>
>> This is why, after doing the GSO support, I decided not to include it.
>
> Gotcha. I don't think it will ever go further than what we have now.
> Placing data into chunks later is not really feasible/wanted,
> especially now with stream schedulers and idata chunks. Doesn't seem
> worth the hassle... we would have to support things like, "segment
> half of this message plus a third of this other one from that other
> stream." (in case it's round robin).
>
>   Marcelo