RE: [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V

[KY Srinivasan]

> -----Original Message-----
> From: David Laight [mailto:David.Laight@xxxxxxxxxx]
> Sent: Thursday, September 17, 2015 1:38 AM
> To: KY Srinivasan <kys@xxxxxxxxxxxxx>; Alexander Duyck
> <alexander.duyck@xxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> Jason Wang <jasowang@xxxxxxxxxx>
> Subject: RE: [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V
> 
> From: KY Srinivasan
> > Sent: 16 September 2015 23:58
> ...
> > > I think we get that.  The question is does the Remote NDIS header and
> > > packet info actually need to be a part of the header data?  I would
> > > argue that it probably doesn't.
> > >
> > > So for example in netvsc_start_xmit it looks like you are calling
> > > init_page_array in order to populate a set of page buffers, but the
> > > first buffer for the Remote NDIS protocol is populated as a separate
> > > page and offset.  As such it doesn't seem like it necessarily needs to
> > > be a part of the header data but could be maintained perhaps in a
> > > separate ring buffer, or perhaps just be a separate page that you break
> > > up to use for each header.
> >
> > You are right; the rndis header can be built as a separate fragment and sent.
> > Indeed this is what we were doing earlier - on the outgoing path we would
> allocate
> > memory for the rndis header. My goal was to avoid this allocation on every
> packet being
> > sent and I decided to use the headroom instead. If we can completely avoid all
> memory
> > allocation for rndis header, it makes a significant perf difference:
> ...
> 
> 
> So just preallocate the header space as a fixed buffer for each ring entry
> (or tx frame).
> 
> If you allocate a fixed buffer for each ring entry you may find there are
> performance gains from copying small fragments into the buffer instead
> of doing whatever mapping operations are required.
> 
> 	David

Yes; I could do that. My original goal of asking for additional head room was to avoid having
any allocation in the transmit path. I did not realize that all I had done was push the allocation to a
different spot since the head room I was asking was greater than the default head room on skb allocation.

I think I can achieve my original goal of not having any allocation in the send path by carefully using the
memory available in the skb:

1. I am going to separately handle the rndis header and this can be packed in the default headroom
available in the skb.
2. I will use the scratch area in the skb to stash away the state that needs to persist. This is the state needed
to cleanup the guest state after we get the send_complete packet.

Regards,

K. Y 
 
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