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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Message-ID: <77823ff9-a02a-808b-e919-b4950a64ae3c@intel.com>
Date:   Thu, 12 Jul 2018 21:20:41 -0400
From:   "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To:     Siwei Liu <loseweigh@...il.com>,
        "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,
        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 7/12/2018 6:19 PM, Siwei Liu wrote:
> 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?

VIRTIO_NET_F_STANDBY as defined in the guest virtio_net driver supports match
by MAC address. I think we should add support for this feature bit in QEMU.
If submitting a patch to update the spec is a pre-requisite to add this
feature bit to QEMU, i can do that.

As far as i understand, group id patches to QEMU are still under review.
Matching by group ID can be another feature bit that could support matching
by group id as well as MAC.


> -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