Re: [RFC] Flexible SR-IOV support for virtio-net

[Yui Washizu]

On 2023/11/18 21:10, Akihiko Odaki wrote:
> Hi,
>
> We are planning to add PCIe SR-IOV support to the virtio-net driver 
> for Windows ("NetKVM")[1], and we want a SR-IOV feature for virtio-net 
> emulation code in QEMU to test it. I expect there are other people 
> interested in such a feature, considering that people are using igb[2] 
> to test SR-IOV support in VMs.
>
> Washizu Yui have already proposed an RFC patch to add a SR-IOV feature 
> to virtio-net emulation[3][4] but it's preliminary and has no 
> configurability for VFs.
>
> Now I'm proposing to add SR-IOV support to virtio-net with full 
> configurability for VFs by following the implementation of virtio-net 
> failover[5]. I'm planning to write patches myself, but I know there 
> are people interested in such patches so I'd like to let you know the 
> idea beforehand.
>
> The idea:
>
> The problem when implementing configurability for VFs is that SR-IOV 
> VFs can be realized and unrealized at runtime with a request from the 
> guest. So a naive implementation cannot deal with a command line like 
> the following:
> -device virtio-net-pci,addr=0x0.0x0,sriov=on
> -device virtio-net-pci,addr=0x0.0x1
> -device virtio-net-pci,addr=0x0.0x2
>
> This will realize the virtio-net functions in 0x0.0x1 and 0x0.0x2 when 
> the guest starts instead of when the guest requests to enable VFs.
>
> However, reviewing the virtio-net emulation code, I realized the 
> virtio-net failover also "hides" devices when the guest starts. The 
> following command line hides hostdev0 when the guest starts, and adds 
> it when the guest requests VIRTIO_NET_F_STANDBY feature:
>
> -device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:6f:55:cc, \
>   bus=root2,failover=on
> -device vfiopci,host=5e:00.2,id=hostdev0,bus=root1,failover_pair_id=net1
>
> So it should be also possible to do similar to "hide" VFs and 
> realize/unrealize them when the guest requests.
>
> There are two things I hate with this idea when contrasting it with 
> the conventional multifunction feature[6] though. One is that the PF 
> must be added before VFs; a similar limitation is imposed for failover.
>
> Another is that it will be specific to virtio-net. I was considering 
> to implement a "generic" SR-IOV feature that will work on various 
> devices, but I realized that will need lots of configuration 
> validations. We may eventually want it, but probably it's better to 
> avoid such a big leap as the first step.
>
> Please tell me if you have questions or suggestions.


Hi, Odaki-san

The idea appears to be practical and convenient.

I have some things I want to confirm.
I understood your idea can make deices for VFs,
created by qdev_new or qdev_realize function, invisible from guest OS.
Is my understanding correct ?
And, if your idea is realized,
will it be possible to specify the backend device for the virtio-pci-net 
device ?

Could you provide insights into the next steps
beyond the implementation details ?
About when do you expect your implementation
to be merged into qemu ?
Do you have a timeline for this plan ?
Moreover, is there any way
we can collaborate on the implementation you're planning ?

Regards,

Yui Washizu


> Regards,
> Akihiko Odaki
>
> [1] https://github.com/virtio-win/kvm-guest-drivers-windows
> [2] https://qemu.readthedocs.io/en/v8.1.0/system/devices/igb.html
> [3] 
> https://patchew.org/QEMU/1689731808-3009-1-git-send-email-yui.washidu@gmail.com/
> [4] 
> https://netdevconf.info/0x17/sessions/talk/unleashing-sr-iov-offload-on-virtual-machines.html
> [5] https://qemu.readthedocs.io/en/v8.1.0/system/virtio-net-failover.html
> [6] https://gitlab.com/qemu-project/qemu/-/blob/v8.1.2/docs/pcie.txt