[PATCH 6/6] net/vhost: perform SW checksum in Tx path

[Ma, WenwuX]

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin at redhat.com>
> Sent: 2022年6月8日 16:14
> To: Ma, WenwuX <wenwux.ma at intel.com>; dev at dpdk.org;
> jasowang at redhat.com; Xia, Chenbo <chenbo.xia at intel.com>;
> david.marchand at redhat.com; Matz, Olivier <olivier.matz at 6wind.com>
> Cc: stable at dpdk.org
> Subject: Re: [PATCH 6/6] net/vhost: perform SW checksum in Tx path
> 
> Hi Wenwu,
> 
> On 6/6/22 11:44, Ma, WenwuX wrote:
> >
> >
> >> -----Original Message-----
> >> From: Maxime Coquelin <maxime.coquelin at redhat.com>
> >> Sent: 2022年6月2日 17:07
> >> To: Ma, WenwuX <wenwux.ma at intel.com>; dev at dpdk.org;
> >> jasowang at redhat.com; Xia, Chenbo <chenbo.xia at intel.com>;
> >> david.marchand at redhat.com; Matz, Olivier <olivier.matz at 6wind.com>
> >> Cc: stable at dpdk.org
> >> Subject: Re: [PATCH 6/6] net/vhost: perform SW checksum in Tx path
> >>
> >> Hi Wenwu,
> >>
> >> Sorry, I missed your review.
> >>
> >> On 5/7/22 05:20, Ma, WenwuX wrote:
> >>>
> >>>> -----Original Message-----
> >>>> From: Maxime Coquelin <maxime.coquelin at redhat.com>
> >>>> Sent: 2022年5月5日 18:27
> >>>> To: dev at dpdk.org; jasowang at redhat.com; Xia, Chenbo
> >>>> <chenbo.xia at intel.com>; david.marchand at redhat.com;
> >>>> olivier.matz at 6wind.com
> >>>> Cc: stable at dpdk.org; Maxime Coquelin
> <maxime.coquelin at redhat.com>
> >>>> Subject: [PATCH 6/6] net/vhost: perform SW checksum in Tx path
> >>>>
> >>>> Virtio specification supports guest checksum offloading for L4,
> >>>> which is enabled with VIRTIO_NET_F_GUEST_CSUM feature
> negotiation.
> >> However,
> >>>> the Vhost PMD does not advertise Tx checksum offload capabilities.
> >>>>
> >>>> Advertising these offload capabilities at the ethdev level is not
> >>>> enough, because we could still end-up with the application enabling
> >>>> these offloads while the guest not negotiating it.
> >>>>
> >>>> This patch advertizes the Tx checksum offload capabilities, and
> >>>> introduces a compatibility layer to cover the case
> >>>> VIRTIO_NET_F_GUEST_CSUM has not been negotiated but the
> >> application
> >>>> does configure the Tx checksum offloads. This function performs the
> >>>> L4 Tx
> >> checksum in SW for UDP and TCP.
> >>>> Compared to Rx SW checksum, the Tx SW checksum function needs to
> >>>> compute the pseudo-header checksum, as we cannot knwo whether it
> >> was
> >>>> done before.
> >>>>
> >>>> This patch does not advertize SCTP checksum offloading capability
> >>>> for now, but it could be handled later if the need arises.
> >>>
> >>> In virtio_enqueue_offload(), if RTE_MBUF_F_TX_IP_CKSUM is set, we
> >>> will performs the L3 Tx checksum, why do not we advertise IPV4
> >>> checksum
> >> offloading capability?
> >>> Will we advertise it later?
> >>>
> >>
> >> Indeed, we have an IPv4 SW checksum fallback in Vhost library.
> >> We could think about adding the capability, but that's not urgent I
> >> think. Do you have a use-case where it is needed?
> >>
> > The GRO/GSO library doesn't re-calculate IPv4 checksums for
> merged/fragmented packets, it will cause iperf in the vm to fail.
> 
> Can you please elaborate?
> 
> If we don't expose the IPv4 checksum availability, it would be done by the
> application using the Vhost PMD, so the result will be the same from the VM
> point of view. Am I missing something?
> 
Ok, it is not urgent.

> Thanks,
> Maxime
> 
> >> Regards,
> >> Maxime
> >