Re: [PATCH net-next 2/2] virtio-net: fix the XDP truesize calculation for mergeable buffers

[Jason Wang]

On 2020/5/6 äå8:08, Michael S. Tsirkin wrote:
> On Wed, May 06, 2020 at 04:21:15PM +0800, Jason Wang wrote:
> > On 2020/5/6 äå3:37, Michael S. Tsirkin wrote:
> > > On Wed, May 06, 2020 at 02:16:33PM +0800, Jason Wang wrote:
> > > > We should not exclude headroom and tailroom when XDP is set. So this
> > > > patch fixes this by initializing the truesize from PAGE_SIZE when XDP
> > > > is set.
> > > >
> > > > Cc: Jesper Dangaard Brouer<brouer@xxxxxxxxxx>
> > > > Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > > Seems too aggressive, we do not use up the whole page for the size.
> > For XDP yes, we do:
> >
> > static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
> >  ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â struct ewma_pkt_len *avg_pkt_len,
> >  ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â unsigned int room)
> > {
> >  ÂÂÂ const size_t hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> >  ÂÂÂ unsigned int len;
> >
> >  ÂÂÂ if (room)
> >  ÂÂÂ ÂÂÂ return PAGE_SIZE - room;
> >
> > ...
> >
> > Thanks
> Hmm. But that's only for new buffers. Buffers that were outstanding
> before xdp was attached don't use the whole page, do they?


They don't and in either case, we've encoded truesize in the ctx. Any 
issue you saw?


> Also, with TCP smallqueues blocking the queue like that might be a problem.
> Could you try and check performance impact of this?


I'm not sure I get you, TCP small queue is more about TX I guess. And 
since we've invalidated the vnet header, the performance of XDP_PASS 
won't be good.


> I looked at what other drivers do and I see they tend to copy the skb
> in XDP_PASS case. ATM we don't normally - but should we?


My understanding is XDP runs before skb, so I don't get here. Or maybe 
you can point me the driver you mentioned here? I've checked i40e and 
mlx5e, both of them build skb after XDP.

Thanks

>