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

[Siwei Liu]

On Fri, Jun 22, 2018 at 3:25 PM, Michael S. Tsirkin <address@hidden> wrote:
> On Fri, Jun 22, 2018 at 02:51:11PM -0700, Siwei Liu wrote:
>> On Fri, Jun 22, 2018 at 2:29 PM, Michael S. Tsirkin <address@hidden> wrote:
>> > On Fri, Jun 22, 2018 at 01:03:04PM -0700, Siwei Liu wrote:
>> >> On Fri, Jun 22, 2018 at 1:00 PM, Siwei Liu <address@hidden> wrote:
>> >> > On Thu, Jun 21, 2018 at 7:32 PM, Michael S. Tsirkin <address@hidden> 
>> >> > 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 <address@hidden> wrote:
>> >> >>> > On Wed, 20 Jun 2018 22:48:58 +0300
>> >> >>> > "Michael S. Tsirkin" <address@hidden> 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
>> >
>> > Let's go back for a bit.
>> >
>> > What is wrong with matching by mac?
>> >
>> > 1. Does not allow multiple NICs with same mac
>> > 2. Requires MAC to be programmed by host in the PT device
>> >    (which is often possible with VFs but isn't possible with all PT
>> >    devices)
>>
>> Might not neccessarily be something wrong, but it's very limited to
>> prohibit the MAC of VF from changing when enslaved by failover.
>
> You mean guest changing MAC? I'm not sure why we prohibit that.

I think Sridhar and Jiri might be better person to answer it. My
impression was that sync'ing the MAC address change between all 3
devices is challenging, as the failover driver uses MAC address to
match net_device internally.

>
>
>> The
>> same as you indicated on the PT device. I suspect the same is
>> prohibited on even virtio-net and failover is because of the same
>> limitation.
>>
>> >
>> > Both issues are up to host: if host needs them it needs the UUID
>> > feature, if not MAC matching is sufficient.
>>
>> I know. What I'm saying is that we rely on STANDBY feature to control
>> the visibility of the primary, not the UUID feature.
>>
>> -Siwei
>
> And what I'm saying is that it's up to a host policy.

So lets say host policy explicitly requires UUID but the guest does
not support it. The guest could be a legacy guest with no STANDBY
support, or a relevately recent guest with STANDBY support but without
the UUID support. In either case, since host asked for UUID matching
explicitly, maybe because it requires different NIC using same MAC, so
it has to override the negotiation result of STANDBY, even if STANDBY
is set and supported? That will end up with inter-feature dependency
over STANDBY, as you see.

-Siwei

>
>> >
>> >
>> >> > 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
>> >