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:   Tue, 10 Jul 2018 11:56:28 -0700
From:   Siwei Liu <loseweigh@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     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 Mon, Jul 9, 2018 at 6:58 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Mon, Jul 09, 2018 at 06:11:53PM -0700, si-wei liu wrote:
>> What do we buy
>> for using a random address during initial discovery and requiring VF to
>> complete the handshake?
>
> I don't see advantages to using a random address that is then
> changed either: changing a MAC causes network downtime for most users.
>
Definitely.

I see Linux host stack fundamentally different with Windows, it's
non-sense to duplicate what Hyper-V is doing especially if there's no
extra benefit.

>> Less network downtime during datapath switching?
>> Sorry but that's not a key factor at all for our main goal - live migration.
>
> Isn't avoiding downtime what makes the migration "live"?
> If you don't care about it at all just remove the device
> and migrate without all these tricks.

Apparently this downtime is not avoidable even if guest initiates the
switch-over when it is done on Linux host stack. Unless the NIC
supports adding duplicate MAC filters with one has higher priority
than the other when both are present. I feel there's very little or
perhaps zero improvement for the downtime if moving to a
guest-initiated datapath switching model.

However, since this downtime is intermittent and generally
unnoticeable with a few packet drops, network should be resilient in
recovering from the drops. My point is that unless we can move to a
datapath switching model with zero downtime theoretically, this kind
of minor optimization offers very little help in general.

Regards,
-Siwei

>
>
> --
> MST
>
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