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

["Ma, WenwuX" <wenwux.ma@intel.com>]

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: 2022年6月2日 17:07
> To: Ma, WenwuX <wenwux.ma@intel.com>; dev@dpdk.org;
> jasowang@redhat.com; Xia, Chenbo <chenbo.xia@intel.com>;
> david.marchand@redhat.com; Matz, Olivier <olivier.matz@6wind.com>
> Cc: stable@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@redhat.com>
> >> Sent: 2022年5月5日 18:27
> >> To: dev@dpdk.org; jasowang@redhat.com; Xia, Chenbo
> >> <chenbo.xia@intel.com>; david.marchand@redhat.com;
> >> olivier.matz@6wind.com
> >> Cc: stable@dpdk.org; Maxime Coquelin <maxime.coquelin@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.

> Regards,
> Maxime