netdev - Re: [virtio-dev] Re: [Qemu-devel] [PATCH v3 0/3] Use of unique identifier for pairing virtio and passthrough devices...

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Date:   Thu, 12 Jul 2018 15:19:37 -0700
From:   Siwei Liu <loseweigh@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Cornelia Huck <cohuck@...hat.com>,
        si-wei liu <si-wei.liu@...cle.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Venu Busireddy <venu.busireddy@...cle.com>,
        Marcel Apfelbaum <marcel@...hat.com>,
        virtio-dev@...ts.oasis-open.org, qemu-devel@...gnu.org,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [virtio-dev] Re: [Qemu-devel] [PATCH v3 0/3] Use of unique
 identifier for pairing virtio and passthrough devices...

On Thu, Jul 12, 2018 at 2:00 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Thu, Jul 12, 2018 at 01:52:53PM -0700, Siwei Liu wrote:
>> The definition is incomplete due to lack of spec. There's no "host"
>> part defined yet in the host-guest interface. If match by MAC is an
>> interface, the same must be done on the host(device) side as well,
>> which has been agreed not the way to go. However, I don't think that's
>> what the author intends to do by interpreting his QEMU patch - it
>> missed the other parts as well, such as the feature negotiation and
>> how it interacts with the paired device.
>>
>> What I said is that match by MAC is just a guest implementation that
>> one can change at any time. We now have the group ID on QEMU, why
>> still sticking to matching by MAC? It shoulnd't be a host-guest
>> interface in the first place anyway.
>
> I think that match by MAC is a simple portable way to match devices.
> E.g. it will work seamlessly with niche things like zPCI. However

That's a good point. I'm not sure if it's a valid assumption that zPCI
should always use the same MAC address as that of virtio. Someone
who's more familiar with the use case may decide and work on that. It
means VFIO device has to take in the MAC address as an identifier to
the "-device vfio-pci,.." QEMU option. I think there's no point to
match device using group ID in QEMU while using MAC in the guest.
Based on that assumption, I'd go with making VIRTIO_NET_F_STANDBY to
match device based on group ID, while someone may come up with another
feature bit later, say VIRTIO_NET_F_STANDBY_BY_MAC when its QEMU
support is available. Would it make sense?

-Siwei

> there are other niche use-cases that aren't addressed by match by MAC
> such as PF pass-through as a primary, and the pci bridge trick addresses
> that at cost of some portability.
>
> So I see no issues supporting both mechanisms, but others on the TC
> might feel differently.
>
> --
> MST