Re: [PATCH] hw/net/net_tx_pkt: Fix virtio header without checksum offloading

[Jason Wang]

On Wed, Mar 27, 2024 at 10:53 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> On 2024/03/27 11:50, Jason Wang wrote:
> > On Tue, Mar 26, 2024 at 3:04 PM Akihiko Odaki <akihiko.odaki@daynix.com> 
> > wrote:
> >>
> >> On 2024/03/26 15:51, Jason Wang wrote:
> >>> On Sun, Mar 24, 2024 at 4:32 PM Akihiko Odaki <akihiko.odaki@daynix.com> 
> >>> wrote:
> >>>>
> >>>> It is incorrect to have the VIRTIO_NET_HDR_F_NEEDS_CSUM set when
> >>>> checksum offloading is disabled so clear the bit. Set the
> >>>> VIRTIO_NET_HDR_F_DATA_VALID bit instead to tell the checksum is valid.
> >>>>
> >>>> TCP/UDP checksum is usually offloaded when the peer requires virtio
> >>>> headers because they can instruct the peer to compute checksum. However,
> >>>> igb disables TX checksum offloading when a VF is enabled whether the
> >>>> peer requires virtio headers because a transmitted packet can be routed
> >>>> to it and it expects the packet has a proper checksum. Therefore, it
> >>>> is necessary to have a correct virtio header even when checksum
> >>>> offloading is disabled.
> >>>>
> >>>> A real TCP/UDP checksum will be computed and saved in the buffer when
> >>>> checksum offloading is disabled. The virtio specification requires to
> >>>> set the packet checksum stored in the buffer to the TCP/UDP pseudo
> >>>> header when the VIRTIO_NET_HDR_F_NEEDS_CSUM bit is set so the bit must
> >>>> be cleared in that case.
> >>>>
> >>>> The VIRTIO_NET_HDR_F_NEEDS_CSUM bit also tells to skip checksum
> >>>> validation. Even if checksum offloading is disabled, it is desirable to
> >>>> skip checksum validation because the checksum is always correct. Use the
> >>>> VIRTIO_NET_HDR_F_DATA_VALID bit to claim the validity of the checksum.
> >>>>
> >>>> Fixes: ffbd2dbd8e64 ("e1000e: Perform software segmentation for 
> >>>> loopback")
> >>>> Buglink: https://issues.redhat.com/browse/RHEL-23067
> >>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> >>>> ---
> >>>>    hw/net/net_tx_pkt.c | 3 +++
> >>>>    1 file changed, 3 insertions(+)
> >>>>
> >>>> diff --git a/hw/net/net_tx_pkt.c b/hw/net/net_tx_pkt.c
> >>>> index 2e5f58b3c9cc..c225cf706513 100644
> >>>> --- a/hw/net/net_tx_pkt.c
> >>>> +++ b/hw/net/net_tx_pkt.c
> >>>> @@ -833,6 +833,9 @@ bool net_tx_pkt_send_custom(struct NetTxPkt *pkt, 
> >>>> bool offload,
> >>>>
> >>>>        if (offload || gso_type == VIRTIO_NET_HDR_GSO_NONE) {
> >>>>            if (!offload && pkt->virt_hdr.flags & 
> >>>> VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> >>>> +            pkt->virt_hdr.flags =
> >>>> +                (pkt->virt_hdr.flags & ~VIRTIO_NET_HDR_F_NEEDS_CSUM) |
> >>>> +                VIRTIO_NET_HDR_F_DATA_VALID;
> >>>
> >>> Why VIRTIO_NET_HDR_F_DATA_VALID is used in TX path?
> >>
> >> On igb, a packet sent from a PCI function may be routed to another
> >> function. The virtio header updated here will be directly provided to
> >> the RX path in such a case.
> >
> > But I meant for example net_tx_pkt_send_custom() is used in
> > e1000e_tx_pkt_send() which is the tx path on the host.
> >
> > VIRTIO_NET_HDR_F_DATA_VALID is not necessary in the tx path.
>
> igb passes igb_tx_pkt_vmdq_callback to net_tx_pkt_send_custom().
> igb_tx_pkt_vmdq_callback() passes the packet to its rx path for loopback.
>

You are right, how about igb_tx_pkt_vmdq_callback()?

We probably need to tweak the name if it is only used in rx path.

Thanks

> Regards,
> Akihiko Odaki
>
> >
> > Thanks
> >
> >>
> >> Regards,
> >> Akihiko Odaki
> >>
> >>>
> >>> Thanks
> >>>
> >>>>                net_tx_pkt_do_sw_csum(pkt, 
> >>>> &pkt->vec[NET_TX_PKT_L2HDR_FRAG],
> >>>>                                      pkt->payload_frags + 
> >>>> NET_TX_PKT_PL_START_FRAG - 1,
> >>>>                                      pkt->payload_len);
> >>>>
> >>>> ---
> >>>> base-commit: ba49d760eb04630e7b15f423ebecf6c871b8f77b
> >>>> change-id: 20240324-tx-c57d3c22ad73
> >>>>
> >>>> Best regards,
> >>>> --
> >>>> Akihiko Odaki <akihiko.odaki@daynix.com>
> >>>>
> >>>
> >>
> >
>