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Re: [libvirt] NFS over AF_VSOCK in <filesystem>



On Tue, May 10, 2016 at 01:03:48PM +0100, Stefan Hajnoczi wrote:
> On Mon, May 09, 2016 at 05:18:42PM +0100, Daniel P. Berrange wrote:
> > On Mon, May 09, 2016 at 04:57:17PM +0100, Stefan Hajnoczi wrote:
> > > virtio-vsock support has been added to the nfs-ganesha NFS server.  I'm
> > > currently working on upstreaming virtio-vsock into Linux and QEMU.  I
> > > also have patches for the Linux NFS client and server.
> > > 
> > > Users wishing to share a file system with the guest will need to
> > > configure the NFS server.  Perhaps libvirt could handle that given that
> > > it already has <filesystem> syntax.
> > > 
> > > The basic task is setting up either the kernel nfsd or nfs-ganesha for
> > > the VM to access the NFS export(s).  When the VM is destroy the NFS
> > > server can be shut down.
> > 
> > Can you elaborate on the interaction between QEMU and the NFS server
> > on the host ? What actually needed changing in nfs-ganesha to support
> > virtio-vsock ?  I thought that on the host side we wouldn't need any
> > changes, because QEMU would just talk to a regular NFS server over
> > TCP, and the only virtio-vsock changes would be in QEMU and the guest
> > kernel.
> 
> The NFS protocol (and SUNRPC) is aware of the transport its running
> over.  In order to fully support the protocol it needs to know about
> AF_VSOCK and addressing.
> 
> The NFS server changes allow specifying an AF_VSOCK listen port.  The
> machine name format in /etc/exports or equivalent config also needs to
> support vsock.

So from host POV, in our current model of exposing host FS to the guest
where libvirt wants control over managing exports, I don't think we
would be inclined to use the in-kernel NFS server at all, nor would we
likely use the main system ganesha server instance.

Instead what I think we'd want is to run an isolated instance of ganesha
for each QEMU VM that requires filesystem passthrough. First of all this
removes any unpredictability around setup, as arbitrary admin config
changes to the default system ganesha server would not conflict with
settings libvirt needed to make for QEMU. Second it would let us place
the ganesha server associated with a VM in the same cgroup, so we can
ensure resources limits associated with the VM get applied. Third it
would let us apply the per-VM svirt MCS level to each ganesha, to
ensure that there's no risk of cross-VM attack vectors via the
ganesha services. Ideally QEMU would talk to ganesha over a private
UNIX domain socket though it looks like ganesha only has the ability
to use TCP or UDP right now, so that'd be something we need to add
support for.

Meanwhile we could also extend the libvirt <filesystem> passthrough
syntax, so that instead of just allowing passthrough from a local
host directory, we could tell it to passthrough straight from some
external server. eg, we could tell it to passthrough straight from
some externally managed ganesha, whether it be on the local host or
on a remote host.

Regards,
Daniel
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