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

[Yui Washizu <yui.washidu@gmail.com>]

On 2024/07/15 14:15, Akihiko Odaki wrote:
> 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.


OK.
I want to use this emulation for offloading virtual network
in a environment where there are many containers in VMs.
So, I consider that the feature is need.
I think that 7 VFs are too few.
I'll keep thinking about the feature's necessity.


I'll add other comments to RFC v5 patch.


Regards,

Yui Washizu