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Date:   Mon, 20 Jan 2020 07:03:20 -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, jhansen@vmware.com,
        jasowang@redhat.com, kvm@vger.kernel.org, stefanha@redhat.com,
        virtualization@lists.linux-foundation.org,
        linux-hyperv@vger.kernel.org, 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 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.

If it does exist, you need a way for userspace to opt-in,
module parameter isn't that.

> 
> > 
> > I am totally against adding a module parameter for this, it's
> > incredibly confusing for users and will create a test scenerio
> > that is strongly less likely to be covered.
> > 
> 
> Got it, I'll remove the module parameter!
> 
> Thanks,
> Stefano