Re: [Qemu-devel] vhost-pci and virtio-vhost-user

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jan 12, 2018 at 02:44:00PM +0800, Wei Wang wrote:
> On 01/11/2018 05:56 PM, Stefan Hajnoczi wrote:
> > On Thu, Jan 11, 2018 at 6:31 AM, Wei Wang <wei.w.wang@intel.com> wrote:
> > > On 01/11/2018 12:14 AM, Stefan Hajnoczi wrote:
> > > Without above two, the solution already works well, so I'm not sure why would we need the above two from functionality point of view.
> > The "[PATCH v3 0/7] Vhost-pci for inter-VM communication" series is
> > incomplete.  It is a subset of vhost-user-net and it works only for
> > poll-mode drivers.  It's the requirements that haven't been covered by
> > the vhost-pci patch series yet that make me prefer the
> > virtio-vhost-user approach.
> > 
> > The virtio device design needs to be capable of supporting the rest of
> > vhost-user functionality in the future.  Once the code is merged in
> > QEMU and DPDK it will be very difficult to make changes to the virtio
> > device.
> 
> This is how virtio works. A new feature with a new feature bit.

Although it is possible to add a feature bit that radically changes the
virtio device's interface, the existing driver software would need to be
largely rewritten - all the way up to the net, scsi, blk, etc devices.

We cannot easily migrate between the vhost-pci to virtio-vhost-user
approaches later.

> Now, we let
> the guest driver join the vhost-user negotiation (including feature
> negotiation), the default device/driver feature negotiation is free to use.
> I'm thinking if it is worthwhile to do some kind of mediated passthrough,
> which passes the selected messages only. Because many messages are not
> necessary to be passed down (e.g. VHOST_USER_SEND_RARP is not needed for
> simple VM-to-VM communication), though might be safe to do. I plan to see
> your full passthrough code first, and see if changing to mediated
> passthrough would be simpler.

I expect vhost-pci to require fewer code changes.  If you judge
"simpler" just by the patch count or size, then vhost-pci will win.

The reason for that is virtio-vhost-user integrates with librte_vhost.
This requires refactoring librte_vhost to support multiple transports.

I think the virtio-vhost-user end result is worth it though: vhost
devices like examples/vhost/ and examples/vhost/scsi/ will work with
both AF_UNIX and virtio-vhost-user.  This makes it simpler for users and
vhost device developers - you only have one implementation of net, scsi,
blk, etc devices.

Stefan

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