Re: [PATCH RFC v4 0/7] virtio-net: add support for SR-IOV emulation

[Akihiko Odaki]

On 2024/05/16 11:00, Yui Washizu wrote:
> On 2024/04/28 18:05, Akihiko Odaki wrote:
> > Based-on: <20240315-reuse-v9-0-67aa69af4d53@daynix.com>
> > ("[PATCH for 9.1 v9 00/11] hw/pci: SR-IOV related fixes and 
> > improvements")
> >
> > Introduction
> > ------------
> >
> > This series is based on the RFC series submitted by Yui Washizu[1].
> > See also [2] for the context.
> >
> > This series enables SR-IOV emulation for virtio-net. It is useful
> > to test SR-IOV support on the guest, or to expose several vDPA devices
> > in a VM. vDPA devices can also provide L2 switching feature for
> > offloading though it is out of scope to allow the guest to configure
> > such a feature.
> >
> > The PF side code resides in virtio-pci. The VF side code resides in
> > the PCI common infrastructure, but it is restricted to work only for
> > virtio-net-pci because of lack of validation.
> >
> > User Interface
> > --------------
> >
> > A user can configure a SR-IOV capable virtio-net device by adding
> > virtio-net-pci functions to a bus. Below is a command line example:
> >    -netdev user,id=n -netdev user,id=o
> >    -netdev user,id=p -netdev user,id=q
> >    -device pcie-root-port,id=b
> >    -device virtio-net-pci,bus=b,addr=0x0.0x3,netdev=q,sriov-pf=f
> >    -device virtio-net-pci,bus=b,addr=0x0.0x2,netdev=p,sriov-pf=f
> >    -device virtio-net-pci,bus=b,addr=0x0.0x1,netdev=o,sriov-pf=f
> >    -device virtio-net-pci,bus=b,addr=0x0.0x0,netdev=n,id=f
> >
> > The VFs specify the paired PF with "sriov-pf" property. The PF must be
> > added after all VFs. It is user's responsibility to ensure that VFs have
> > function numbers larger than one of the PF, and the function numbers
> > have a consistent stride.
>
>
> I tried to start a VM with more than 8 VFs allocated using your patch,
> but the following error occured and qemu didn't work:
> VF function number overflows.
>
> I think the cause of this error is that virtio-net-pci PFs don't have ARI.
> (pcie_ari_init is not added to virtio-net-pci when PFs are initialized.)
> I think it is possible to add it later,
> but how about adding pcie_ari_init ?
>
> As a trial,
> adding pcie_ari_init to virtio_pci_realize enabled the creation of more 
> than 8 VFs.

I have just looked into that possibility, but adding pcie_ari_init to 
virtio_pci_realize has some implications. Unconditionally calling 
pcie_ari_init will break the existing configuration of virtio-pci 
devices so we need to implement some logic to detect when ARI is needed. 
Preferably such logic should be implemented in the common PCI 
infrastructure instead of implementing it in virtio-pci so that other 
PCI multifunction devices can benefit from it.

While I don't think implementing this will be too complicated, I need to 
ensure that such a feature is really needed before doing so.