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 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?

> 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?

-- 
MST