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

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

On Mon, Jan 09, 2017 at 06:46:25PM +0100, Olivier Matz wrote:
> The virtio specifications requires that the L4 checksum is set to the
> pseudo header checksum. You can search for "pseudo header" in the
> following doc:
> http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.pdf
> 
> Especially in 5.1.6.2.1, we can see that if we use the csum flag, we
> must set the checksum to phdr, and if we do tso, we must set the csum
> flag.
> 
> We can check that this is really needed with Linux vhost by replaying
> the test plan described at [1].
> 
> [1] http://dpdk.org/ml/archives/dev/2016-October/048793.html
> 
> If we add the following patch to disable the checksum fix (on top of
> this patchset), the test1 "large packets (lro/tso)" won't work.
> 
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -224,6 +224,9 @@
>         uint32_t tmp;
>         int shared = 0;
>  
> +        if (1)
> +               return 0;
> +
>         /* mbuf is write-only, we need to copy the headers in a linear
> buffer */ if (unlikely(rte_pktmbuf_data_is_shared(m, 0, hdrlen))) {
>                 shared = 1;
> 
> 
> In one direction ("flow1" in the test desc), large packets are
> transmitted from host on the ixgbe interface, and received by the
> guest. Then, testpmd bridges the packet to the virtio interface. But
> the packet is not received by the host.

I hope I could have time to dig this further, since, honestly, I don't
quite like this patch: it makes things un-maintainable.

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").

Cc more people here. And here is a quick background for them: NIC drivers
doing TSO need change the mbuf (say, for cksum updating), however, as
Stephen pointed out, we could not do that if the mbuf is shared: I don't
see such checks in the driver code as well.

> There are at least 2 options for this one:
> 
> - try to use 2 different descriptors (the patch is probably harder,
>   and it may slow-down the case where ANY_LAYOUT is supported)
> 
> - refuse to initialize with TSO enabled if ANY_LAYOUT is not supported.
> 
> If you think ANY_LAYOUT is most likely true today, we could choose
> option 2. Let me know what's your preference here.

Maybe we could go with a simpler one: COW. Yeah, it costs more, but this
would be rare, that it should be OKay, right? Besides, we just need copy
the heading mbuf.

	--yliu