Re: [virtio-comment] [PATCH] virtio-net: Add support for correct hdr_len field.

[Vitaly Mireyno <vmireyno@marvell.com>]

>-----Original Message-----
>From: Michael S. Tsirkin <mst@redhat.com>
>Sent: Thursday, 20 February, 2020 12:01
>To: Vitaly Mireyno <vmireyno@marvell.com>
>Cc: virtio-comment@lists.oasis-open.org; jasowang@redhat.com
>Subject: [EXT] Re: [virtio-comment] [PATCH] virtio-net: Add support for correct hdr_len field.
>
>----------------------------------------------------------------------
>On Thu, Feb 20, 2020 at 09:51:17AM +0000, Vitaly Mireyno wrote:
>>
>> >-----Original Message-----
>> >From: virtio-comment@lists.oasis-open.org
>> ><virtio-comment@lists.oasis-open.org> On Behalf Of Michael S. Tsirkin
>> >Sent: Thursday, 20 February, 2020 10:12
>> >To: Vitaly Mireyno <vmireyno@marvell.com>
>> >Cc: virtio-comment@lists.oasis-open.org; jasowang@redhat.com
>> >Subject: Re: [virtio-comment] [PATCH] virtio-net: Add support for correct hdr_len field.
>> >
>> >---------------------------------------------------------------------
>> >- On Thu, Oct 24, 2019 at 03:24:43PM +0000, Vitaly Mireyno wrote:
>> >> Some devices benefit from the knowledge of the exact header length for TSO processing.
>> >> Add a feature bit for a driver that is capable of providing the correct header length for TSO
>packets.
>> >>
>> >> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
>> >
>> >So I looked at implementing this and maybe this was not such a good idea after all.
>> >
>> >How would we implement this in Linux?
>> >Current code just puts skb_headlen there which is # of bytes in the linear buffer.
>> >Which I guess it often the header, but not at all always.
>> >
>> >Should this have been limited to TSO packets?
>> >
>> >I also could not figure out how this is useful for the host.
>> >Could you enlighten me pls?
>>
>> As see it, header length is essential for TSO, and maybe not so useful for non-TSO.
>
>So maybe we should limit this for when gso type is actually set?

Do you mean the hdr_len field will be valid only for TSO packets, or it will be accurate only for TSO packets?
I'm fine with both options.

>
>> To calculate the header length, I suppose a Linux driver could do something like:
>> skb_transport_header(skb) + tcp_hdrlen(skb) - skb->data
>
>That's parsing the header in software. Why not have the card do it in hardware?
>

It depends on hw architecture. The architecture I'm familiar with, the hw can parse the header, but it happens too late for the segmentation decision.


>--
>MST