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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 20, 2020 at 10:18:32AM +0000, Vitaly Mireyno wrote:
> 
> >-----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.

Let's say accurate only for TSO.

Can you write a spec patch like this?

> >
> >> 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 only good for TCP though.


> >
> >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