RE: Questions about RX checksum offload on virtio and vhost

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Morten Brørup
> Sent: Tuesday, 15 September 2026 11.57
> 
> Hi Maxime,
> 
> A Grout issue [1] regarding RX checksum offload with virtio and vhost
> triggered my curiosity...
> 
> AFAIU, in Linux, the VIRTIO_NET_F_GUEST_CSUM flag on a virtio network
> device tells the host that it doesn't need to set the correct checksums
> in packets it writes to the device, because the guest will do it (when
> reading them).
> 
> Now; if a guest application requests RTE_ETH_RX_OFFLOAD_CHECKSUM
> offload, the virtio PMD initializes the Linux device with
> VIRTIO_NET_F_GUEST_CSUM [2].
> This behavior seems counterintuitive to me...
> I would assume that if an application cares about packet checksums on
> ingress, it will set RTE_ETH_RX_OFFLOAD_CHECKSUM, so the NIC/PMD checks
> them, and the application can discard packets with invalid checksums on
> ingress.
> But the virtio PMD uses RTE_ETH_RX_OFFLOAD_CHECKSUM to disable
> checksums, not enable them?
> 
> Two questions:
> 1. What is the use case for a guest application receiving packets
> without correct packet checksums from the virtio ports, which motivated
> this driver behavior?
> 2. What is the performance gain in the Linux host by not setting the
> checksums (enabling VIRTIO_NET_F_GUEST_CSUM)? (If the guest application
> needs to calculate them instead, the performance cost in the guest
> might outweigh the performance gain in the host not doing it when
> creating the packet in the Linux kernel.)
> 
> NB: According to the Grout issue, the vhost PMD behaves similarly; but
> I haven't checked.
> 
> [1]: https://github.com/DPDK/grout/issues/724
> [2]: https://github.com/DPDK/dpdk/blob/25.11/drivers/net/virtio/virtio_ethdev.c#L2282-L2284
> 

I came across a patch [3], which writes into the packet's checksum field.
It seems to be for the virtio PMD running on the host.
Is something similar missing for the virtio PMD running in a guest?

https://github.com/DPDK/dpdk/commit/8f1750f42e2d2dbf49af59eb058e8412f0e3afa4