Re: [dpdk-dev] [PATCH 4/4] net/virtio: improve offload check performance

[Tiwei Bie <tiwei.bie@intel.com>]

On Tue, Jun 05, 2018 at 11:43:11AM +0200, Maxime Coquelin wrote:
> On 06/05/2018 05:10 AM, Tiwei Bie wrote:
> > On Mon, Jun 04, 2018 at 04:29:56PM +0200, Maxime Coquelin wrote:
> > > On 06/04/2018 01:55 PM, Tiwei Bie wrote:
> > > > On Fri, Jun 01, 2018 at 02:47:58PM +0200, Maxime Coquelin wrote:
> > [...]
> > > > > @@ -253,13 +246,15 @@ virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie,
> > > > >    	struct virtio_net_hdr *hdr;
> > > > >    	int offload;
> > > > > -	offload = tx_offload_enabled(vq->hw);
> > > > >    	head_idx = vq->vq_desc_head_idx;
> > > > >    	idx = head_idx;
> > > > >    	dxp = &vq->vq_descx[idx];
> > > > >    	dxp->cookie = (void *)cookie;
> > > > >    	dxp->ndescs = needed;
> > > > > +	offload = vq->hw->has_tx_offload &&
> > > > > +		(cookie->ol_flags & PKT_TX_OFFLOAD_MASK);
> > > > 
> > > > If features aren't negotiated, I think there is no need to
> > > > check ol_flags and update the net header.
> > > 
> > > Isn't what the code is doing?
> > > has_tx_offload will be false if none of the Tx offload features have
> > > been negotiated, so ol_flags won't be checked in that case.
> > 
> > Hmm.. Somehow I treated 'and' as 'or'..
> > 
> > I have another question. When 'can_push' is false
> > and 'vq->hw->has_tx_offload' is true, there will
> > be a chance that virtio net hdr won't be zeroed
> > when ol_flags doesn't specify any Tx offloads.
> 
> Right, good catch.
> It may be better to remove this small optimization.
> Indeed, with the series, if the application does not enable offloads,
> then the Virtio features are re-negotiated with the offload features.

Yeah. It's a good idea to disable the features when
the corresponding Tx offloads aren't requested by
the applications! I like it!

This issue happens for the mbufs whose ol_flags
doesn't specify Tx offloads when applications
enable Tx offloads and can_push is false. I think
when applications enable Tx offloads, although
most packets to be sent will have Tx offloads
specified in their ol_flags, it's still possible
that some packets don't have Tx offloads specified
in their ol_flags.

Best regards,
Tiwei Bie

> 
> Thanks,
> Maxime
> 
> > Best regards,
> > Tiwei Bie
> >