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

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

On Tue, Jun 05, 2018 at 01:58:00PM +0200, Maxime Coquelin wrote:
> On 06/05/2018 01:20 PM, Tiwei Bie wrote:
> > 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.
> 
> Reading again my reply, I think it wasn't clear enough,
> let me rephrase it.
> 
> My proposal is to keep disabling the features if the corresponding
> Tx offloads aren't negotiated by the application, but just to remove the
> check on mbuf's ol_flags, so:
> offload = vq->hw->has_tx_offload;
> 
> Doing that, we retrieve the old behaviour, i.e. if Virtio features are
> negotiated but no ol_flags set, virtio-net header will be cleared.

It sounds good! Thanks!

Best regards,
Tiwei Bie