[dpdk-dev] [PATCH 5/5] net/virtio: fix Tso when mbuf is shared

[Olivier MATZ]

On Wed, 18 Jan 2017 13:03:48 +0800, Yuanhan Liu
<yuanhan.liu at 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.

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?


Thanks
Olivier