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Date:   Mon, 20 Jan 2020 11:04:00 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     David Miller <davem@davemloft.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Jorgen Hansen <jhansen@vmware.com>,
        Jason Wang <jasowang@redhat.com>, kvm <kvm@vger.kernel.org>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-hyperv@vger.kernel.org, Dexuan Cui <decui@microsoft.com>
Subject: Re: [PATCH net-next 1/3] vsock: add network namespace support
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On Mon, Jan 20, 2020 at 02:58:01PM +0100, Stefano Garzarella wrote:
> On Mon, Jan 20, 2020 at 1:03 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Mon, Jan 20, 2020 at 11:17:35AM +0100, Stefano Garzarella wrote:
> > > On Mon, Jan 20, 2020 at 10:06:10AM +0100, David Miller wrote:
> > > > From: Stefano Garzarella <sgarzare@redhat.com>
> > > > Date: Thu, 16 Jan 2020 18:24:26 +0100
> > > >
> > > > > This patch adds 'netns' module param to enable this new feature
> > > > > (disabled by default), because it changes vsock's behavior with
> > > > > network namespaces and could break existing applications.
> > > >
> > > > Sorry, no.
> > > >
> > > > I wonder if you can even design a legitimate, reasonable, use case
> > > > where these netns changes could break things.
> > >
> > > I forgot to mention the use case.
> > > I tried the RFC with Kata containers and we found that Kata shim-v1
> > > doesn't work (Kata shim-v2 works as is) because there are the following
> > > processes involved:
> > > - kata-runtime (runs in the init_netns) opens /dev/vhost-vsock and
> > >   passes it to qemu
> > > - kata-shim (runs in a container) wants to talk with the guest but the
> > >   vsock device is assigned to the init_netns and kata-shim runs in a
> > >   different netns, so the communication is not allowed
> > > But, as you said, this could be a wrong design, indeed they already
> > > found a fix, but I was not sure if others could have the same issue.
> > >
> > > In this case, do you think it is acceptable to make this change in
> > > the vsock's behavior with netns and ask the user to change the design?
> >
> > David's question is what would be a usecase that's broken
> > (as opposed to fixed) by enabling this by default.
> 
> Yes, I got that. Thanks for clarifying.
> I just reported a broken example that can be fixed with a different
> design (due to the fact that before this series, vsock devices were
> accessible to all netns).
> 
> >
> > If it does exist, you need a way for userspace to opt-in,
> > module parameter isn't that.
> 
> Okay, but I honestly can't find a case that can't be solved.
> So I don't know whether to add an option (ioctl, sysfs ?) or wait for
> a real case to come up.
> 
> I'll try to see better if there's any particular case where we need
> to disable netns in vsock.
> 
> Thanks,
> Stefano

Me neither. so what did you have in mind when you wrote:
"could break existing applications"?