Re: [virtio-dev] Re: [Qemu-devel] [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net

[Siwei Liu <loseweigh@gmail.com>]

On Fri, Jun 22, 2018 at 1:00 PM, Siwei Liu <loseweigh@gmail.com> wrote:
> On Thu, Jun 21, 2018 at 7:32 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> On Thu, Jun 21, 2018 at 06:21:55PM -0700, Siwei Liu wrote:
>>> On Thu, Jun 21, 2018 at 7:59 AM, Cornelia Huck <cohuck@redhat.com> wrote:
>>> > On Wed, 20 Jun 2018 22:48:58 +0300
>>> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>> >
>>> >> On Wed, Jun 20, 2018 at 06:06:19PM +0200, Cornelia Huck wrote:
>>> >> > In any case, I'm not sure anymore why we'd want the extra uuid.
>>> >>
>>> >> It's mostly so we can have e.g. multiple devices with same MAC
>>> >> (which some people seem to want in order to then use
>>> >> then with different containers).
>>> >>
>>> >> But it is also handy for when you assign a PF, since then you
>>> >> can't set the MAC.
>>> >>
>>> >
>>> > OK, so what about the following:
>>> >
>>> > - introduce a new feature bit, VIRTIO_NET_F_STANDBY_UUID that indicates
>>> >   that we have a new uuid field in the virtio-net config space
>>> > - in QEMU, add a property for virtio-net that allows to specify a uuid,
>>> >   offer VIRTIO_NET_F_STANDBY_UUID if set
>>> > - when configuring, set the property to the group UUID of the vfio-pci
>>> >   device
>>>
>>> If feature negotiation fails on VIRTIO_NET_F_STANDBY_UUID, is it safe
>>> to still expose UUID in the config space on virtio-pci?
>>
>>
>> Yes but guest is not supposed to read it.
>>
>>> I'm not even sure if it's sane to expose group UUID on the PCI bridge
>>> where the corresponding vfio-pci device attached to for a guest which
>>> doesn't support the feature (legacy).
>>>
>>> -Siwei
>>
>> Yes but you won't add the primary behind such a bridge.
>
> I assume the UUID feature is a new one besides the exiting
> VIRTIO_NET_F_STANDBY feature, where I think the current proposal is,
> if UUID feature is present and supported by guest, we'll do pairing
> based on UUID; if UUID feature present
                                 ^^^^^^^  is NOT present

> or not supported by guest,
> we'll still plug in the VF (if guest supports the _F_STANDBY feature)
> but the pairing will be fallback to using MAC address. Are you saying
> you don't even want to plug in the primary when the UUID feature is
> not supported by guest? Does the feature negotiation UUID have to
> depend on STANDBY being set, or the other way around? I thought that
> just the absence of STANDBY will prevent primary to get exposed to the
> guest.
>
> -Siwei
>
>>
>>>
>>> > - in the guest, use the uuid from the virtio-net device's config space
>>> >   if applicable; else, fall back to matching by MAC as done today
>>> >
>>> > That should work for all virtio transports.