Re: [PATCH 5/5] net/virtio: fix Tso when mbuf is shared

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Tue, Jan 24, 2017 at 11:51:20AM +0100, Olivier MATZ wrote:
> On Wed, 18 Jan 2017 13:03:48 +0800, Yuanhan Liu
> <yuanhan.liu@linux.intel.com> wrote:
> > On Tue, Jan 17, 2017 at 12:18:25PM +0100, Olivier Matz wrote:
> > > > I hope I could have time to dig this further, since, honestly, I
> > > > don't quite like this patch: it makes things un-maintainable.  
> > > 
> > > Well, I'm not that proud of the patch, but that's the best solution
> > > I've found. Nevertheless saying it makes things un-maintainable
> > > looks a bit excessive to me :)  
> > 
> > Aha... really sorry about that!
> > 
> > But honestly, I'd say again, it makes thing more complex, just for
> > fixing a corner and rare issue. I'd try to avoid that.
> > 
> > > 
> > > The option of reallocating a mbuf, copy and fix network headers in
> > > it looks even more complex to me (that was my first approach).
> > >   
> > > > Besides that, I think we have similar issue with nic drivers. See
> > > > the rte_net_intel_cksum_flags_prepare() function introduced at
> > > > commit 4fb7e803eb1a ("ethdev: add Tx preparation").  
> > > 
> > > Yes, that was discussed a bit. See [1] and the subsequent mails.
> > > http://dpdk.org/ml/archives/dev/2016-December/051014.html  
> > 
> > Thanks for the info, and I'm pretty Okay with that.
> > 
> > > My opinion is that tx_burst() should not change the mbuf data, it's
> > > always been like this. For Intel NICs, there is no issue since the
> > > DPDK API is derived from Intel NICs API, so there is no fix to do
> > > in the mbuf data.
> > > 
> > > For tx_prepare(), it's explicitly said that it can update the data.
> > > If tx_prepare() becomes mandatory, it will naturally fix this issue
> > > without modifying the driver, because the phdr csum calculation
> > > will be done in tx_prepare().
> > > 
> > > An alternative is to mark this as a known issue for now, and wait
> > > until tx_prepare() is mandatory.  
> > 
> > I see no reason to wait. Though my understanding is it may not be a
> > mandatory so far, but user is supposed to calculate the pseudo-header
> > checksum by themself before. Now they have one more option:
> > tx_prepare.
> >
> > That means, in either way, user has to do some extra works to make
> > TSO work (either by themself or call tx_prepare). So I don't think
> > it'd be an issue?
> 
> Yes sounds good. I'll check how a tx_prepare() could be implemented for
> virtio, in order to fix this issue.

Thanks.

> In the meanwhile, for the 17.02, I think it could be good to highlight
> the problem in the known issues, what do you think?

Yes, I think it's a good idea.

	--yliu