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

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2024/03/27 11:59, Jason Wang wrote:
> 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.

igb_tx_pkt_vmdq_callback() itself is part of the tx path of a PCI 
function, and invokes the rx path of another PCI function in case of 
loopback, or triggers the transmission to the external peer.

Regards,
Akihiko Odaki

> 
> 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>
>>>>>>
>>>>>
>>>>
>>>
>>
>