Date:   Fri, 21 Apr 2017 12:05:15 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     vyasevic@...hat.com, Vladislav Yasevich <vyasevich@...il.com>,
        netdev@...r.kernel.org
Cc:     virtio-dev@...ts.oasis-open.org, mst@...hat.com,
        maxime.coquelin@...hat.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC (resend) net-next 0/6] virtio-net: Add support for
 virtio-net header extensions



On 2017年04月20日 23:34, Vlad Yasevich wrote:
> On 04/17/2017 11:01 PM, Jason Wang wrote:
>>
>> On 2017年04月16日 00:38, Vladislav Yasevich wrote:
>>> Curreclty virtion net header is fixed size and adding things to it is rather
>>> difficult to do.  This series attempt to add the infrastructure as well as some
>>> extensions that try to resolve some deficiencies we currently have.
>>>
>>> First, vnet header only has space for 16 flags.  This may not be enough
>>> in the future.  The extensions will provide space for 32 possbile extension
>>> flags and 32 possible extensions.   These flags will be carried in the
>>> first pseudo extension header, the presense of which will be determined by
>>> the flag in the virtio net header.
>>>
>>> The extensions themselves will immidiately follow the extension header itself.
>>> They will be added to the packet in the same order as they appear in the
>>> extension flags.  No padding is placed between the extensions and any
>>> extensions negotiated, but not used need by a given packet will convert to
>>> trailing padding.
>> Do we need a explicit padding (e.g an extension) which could be controlled by each side?
> I don't think so.  The size of the vnet header is set based on the extensions negotiated.
> The one part I am not crazy about is that in the case of packet not using any extensions,
> the data is still placed after the entire vnet header, which essentially adds a lot
> of padding.  However, that's really no different then if we simply grew the vnet header.
>
> The other thing I've tried before is putting extensions into their own sg buffer, but that
> made it slower.h

Yes.

>
>>> For example:
>>>    | vnet mrg hdr | ext hdr | ext 1 | ext 2 | ext 5 | .. pad .. | packet data |
>> Just some rough thoughts:
>>
>> - Is this better to use TLV instead of bitmap here? One advantage of TLV is that the
>> length is not limited by the length of bitmap.
> but the disadvantage is that we add at least 4 bytes per extension of just TL data.  That
> makes this thing even longer.

Yes, and it looks like the length is still limited by e.g the length of T.

>
>> - For 1.1, do we really want something like vnet header? AFAIK, it was not used by modern
>> NICs, is this better to pack all meta-data into descriptor itself? This may need a some
>> changes in tun/macvtap, but looks more PCIE friendly.
> That would really be ideal and I've looked at this.  There are small issues of exposing
> the 'net metadata' of the descriptor to taps so they can be filled in.  The alternative
> is to use a different control structure for tap->qemu|vhost channel (that can be
> implementation specific) and have qemu|vhost populate the 'net metadata' of the descriptor.

Yes, this needs some thought. For vhost, things looks a little bit 
easier, we can probably use msg_control.

Thanks

> Thanks
> -vlad
>
>> Thanks
>>
>>> Extensions proposed in this series are:
>>>    - IPv6 fragment id extension
>>>      * Currently, the guest generated fragment id is discarded and the host
>>>        generates an IPv6 fragment id if the packet has to be fragmented.  The
>>>        code attempts to add time based perturbation to id generation to make
>>>        it harder to guess the next fragment id to be used.  However, doing this
>>>        on the host may result is less perturbation (due to differnet timing)
>>>        and might make id guessing easier.  Ideally, the ids generated by the
>>>        guest should be used.  One could also argue that we a "violating" the
>>>        IPv6 protocol in the if the _strict_ interpretation of the spec.
>>>
>>>    - VLAN header acceleration
>>>      * Currently virtio doesn't not do vlan header acceleration and instead
>>>        uses software tagging.  One of the first things that the host will do is
>>>        strip the vlan header out.  When passing the packet the a guest the
>>>        vlan header is re-inserted in to the packet.  We can skip all that work
>>>        if we can pass the vlan data in accelearted format.  Then the host will
>>>        not do any extra work.  However, so far, this yeilded a very small
>>>        perf bump (only ~1%).  I am still looking into this.
>>>
>>>    - UDP tunnel offload
>>>      * Similar to vlan acceleration, with this extension we can pass additional
>>>        data to host for support GSO with udp tunnel and possible other
>>>        encapsulations.  This yeilds a significant perfromance improvement
>>>       (still testing remote checksum code).
>>>
>>> An addition extension that is unfinished (due to still testing for any
>>> side-effects) is checksum passthrough to support drivers that set
>>> CHECKSUM_COMPLETE.  This would eliminate the need for guests to compute
>>> the software checksum.
>>>
>>> This series only takes care of virtio net.  I have addition patches for the
>>> host side (vhost and tap/macvtap as well as qemu), but wanted to get feedback
>>> on the general approach first.
>>>
>>> Vladislav Yasevich (6):
>>>     virtio-net: Remove the use the padded vnet_header structure
>>>     virtio-net: make header length handling uniform
>>>     virtio_net: Add basic skeleton for handling vnet header extensions.
>>>     virtio-net: Add support for IPv6 fragment id vnet header extension.
>>>     virtio-net: Add support for vlan acceleration vnet header extension.
>>>     virtio-net: Add support for UDP tunnel offload and extension.
>>>
>>>    drivers/net/virtio_net.c        | 132 +++++++++++++++++++++++++++++++++-------
>>>    include/linux/skbuff.h          |   5 ++
>>>    include/linux/virtio_net.h      |  91 ++++++++++++++++++++++++++-
>>>    include/uapi/linux/virtio_net.h |  38 ++++++++++++
>>>    4 files changed, 242 insertions(+), 24 deletions(-)
>>>

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