netdev - Re: [PATCH] virtio: don't set VIRTIO_NET_HDR_F_DATA

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Date:   Wed, 18 Jan 2017 17:18:08 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Rolf Neugebauer <rolf.neugebauer@...ker.com>,
        netdev@...r.kernel.org, davem@...emloft.net,
        rppt@...ux.vnet.ibm.com
Subject: Re: [PATCH] virtio: don't set VIRTIO_NET_HDR_F_DATA_VALID on xmit

On Wed, Jan 18, 2017 at 11:54:21AM +0800, Jason Wang wrote:
> 
> 
> On 2017年01月18日 02:27, Michael S. Tsirkin wrote:
> > On Tue, Jan 17, 2017 at 06:13:51PM +0000, Rolf Neugebauer wrote:
> > > This patch part reverts fd2a0437dc33 and e858fae2b0b8 which introduced a
> > > subtle change in how the virtio_net flags are derived from the SKBs
> > > ip_summed field.
> > > 
> > > With the above commits, the flags are set to VIRTIO_NET_HDR_F_DATA_VALID
> > > when ip_summed == CHECKSUM_UNNECESSARY, thus treating it differently to
> > > ip_summed == CHECKSUM_NONE, which should be the same.
> > > 
> > > Further, the virtio spec 1.0 / CS04 explicitly says that
> > > VIRTIO_NET_HDR_F_DATA_VALID must not be set by the driver.
> > > 
> > > Signed-off-by: Rolf Neugebauer <rolf.neugebauer@...ker.com>
> > Fixes: fd2a0437dc33 ("virtio_net: introduce virtio_net_hdr_{from,to}_skb")
> > Fixes: e858fae2b0b8 (" virtio_net: use common code for virtio_net_hdr and skb GSO conversion")
> > Acked-by: Michael S. Tsirkin <mst@...hat.com>
> > 
> > Should be backported into stable as well.
> 
> Looks like a side effect is that we will never see this on receive path?

I thought receive path is using virtio_net_hdr_to_skb - isn't it?

> We
> probably need a hint for virtio_net_hdr_from_skb().
> 
> Thanks
> 
> > 
> > 
> > > ---
> > >   include/linux/virtio_net.h | 2 --
> > >   1 file changed, 2 deletions(-)
> > > 
> > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > > index 66204007d7ac..56436472ccc7 100644
> > > --- a/include/linux/virtio_net.h
> > > +++ b/include/linux/virtio_net.h
> > > @@ -91,8 +91,6 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
> > >   				skb_checksum_start_offset(skb));
> > >   		hdr->csum_offset = __cpu_to_virtio16(little_endian,
> > >   				skb->csum_offset);
> > > -	} else if (skb->ip_summed == CHECKSUM_UNNECESSARY) {
> > > -		hdr->flags = VIRTIO_NET_HDR_F_DATA_VALID;
> > >   	} /* else everything is zero */
> > >   	return 0;
> > > -- 
> > > 2.11.0