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 22:18:04 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Siwei Liu <loseweigh@...il.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 Wed, Jul 11, 2018 at 11:53:44AM +0200, Cornelia Huck wrote:
> On Tue, 10 Jul 2018 17:07:37 -0700
> Siwei Liu <loseweigh@...il.com> wrote:
> 
> > On Mon, Jul 9, 2018 at 6:54 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > > On Mon, Jul 09, 2018 at 06:11:53PM -0700, si-wei liu wrote:  
> > >> The plan is to enable group ID based matching in the first place rather than
> > >> match by MAC, the latter of which is fragile and problematic.  
> > >
> > > It isn't all that fragile - hyperv used same for a while, so if someone
> > > posts working patches with QEMU support but before this grouping stuff,
> > > I'll happily apply them.  
> > 
> > I wouldn't box the solution to very limited scenario just because of
> > matching by MAC, the benefit of having generic group ID in the first
> > place is that we save the effort of maintaining legacy MAC based
> > pairing that just adds complexity anyway. Currently the VF's MAC
> > address cannot be changed by either PF or by the guest user is a
> > severe limitation due to this. The other use case is that PT device
> > than VF would generally have different MAC than the standby virtio. We
> > shouldn't limit itself to VF specific scenario from the very
> > beginning.
> 
> So, this brings me to a different concern: the semantics of
> VIRTIO_NET_F_STANDBY.
> 
> * The currently sole user seems to be the virtio-net Linux driver.
> * The commit messages, code comments and Documentation/ all talk about
>   matching by MAC.
> * I could not find any proposed update to the virtio spec. (If there
>   had been an older proposal with a different feature name, it is not
>   discoverable.)
> 
> VIRTIO_NET_F_STANDBY is a host <-> guest interface. As there's no
> official spec, you can only go by the Linux implementation, and by that
> its semantics seem to be 'match by MAC', not 'match by other criteria'.
> 
> How is this supposed to work in the long run?

We definitely need a spec patch for VIRTIO_NET_F_STANDBY documenting existing
semantics.  Sridhar, do you plan to take a look?

-- 
MST