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Re: [RFC v4 PATCH 00/49] Initial support of multi-process qemu - status

From: Stefan Hajnoczi
Subject: Re: [RFC v4 PATCH 00/49] Initial support of multi-process qemu - status update
Date: Fri, 3 Jan 2020 15:59:20 +0000

On Thu, Jan 02, 2020 at 11:03:22AM +0000, Felipe Franciosi wrote:
> > On Jan 2, 2020, at 10:42 AM, Stefan Hajnoczi <address@hidden> wrote:
> > On Fri, Dec 20, 2019 at 10:22:37AM +0000, Daniel P. Berrangé wrote:
> >> On Fri, Dec 20, 2019 at 09:47:12AM +0000, Stefan Hajnoczi wrote:
> >>> On Thu, Dec 19, 2019 at 12:55:04PM +0000, Daniel P. Berrangé wrote:
> >>>> On Thu, Dec 19, 2019 at 12:33:15PM +0000, Felipe Franciosi wrote:
> >>>>>> On Dec 19, 2019, at 11:55 AM, Stefan Hajnoczi <address@hidden> wrote:
> >>>>>> On Tue, Dec 17, 2019 at 10:57:17PM +0000, Felipe Franciosi wrote:
> >>>>>>>> On Dec 17, 2019, at 5:33 PM, Stefan Hajnoczi <address@hidden> wrote:
> >>>>>>>> On Mon, Dec 16, 2019 at 07:57:32PM +0000, Felipe Franciosi wrote:
> >>>>>>>>>> On 16 Dec 2019, at 20:47, Elena Ufimtseva <address@hidden> wrote:
> >>>>>>>>>> On Fri, Dec 13, 2019 at 10:41:16AM +0000, Stefan Hajnoczi wrote:
> >>>>> To be clear: I'm very happy to have a userspace-only option for this,
> >>>>> I just don't want to ditch the kernel module (yet, anyway). :)
> >>>> 
> >>>> If it doesn't create too large of a burden to support both, then I think
> >>>> it is very desirable. IIUC, this is saying a kernel based solution as the
> >>>> optimized/optimal solution, and userspace UNIX socket based option as the
> >>>> generic "works everywhere" fallback solution.
> >>> 
> >>> I'm slightly in favor of the kernel implementation because it keeps us
> >>> better aligned with VFIO.  That means solving problems in one place only
> >>> and less reinventing the wheel.
> >>> 
> >>> Knowing that a userspace implementation is possible is a plus though.
> >>> Maybe that option will become attractive in the future and someone will
> >>> develop it.  In fact, a userspace implementation may be a cool Google
> >>> Summer of Code project idea that I'd like to co-mentor.
> >> 
> >> If it is technically viable as an approach, then I think  we should be
> >> treating a fully unprivileged muser-over-UNIX socket as a higher priority
> >> than just "maybe a GSoC student will want todo it".
> >> 
> >> Libvirt is getting strong message from KubeVirt project that they want to
> >> be running both libvirtd and QEMU fully unprivileged. This allows their
> >> containers to be unprivileged. Anything that requires privileges requires
> >> jumping through extra hoops writing custom code in KubeVirt to do things
> >> outside libvirt in side loaded privileged containers and this limits how
> >> where those features can be used.
> > 
> > Okay this makes sense.
> > 
> > There needs to be a consensus on whether to go with a qdev-over-socket
> > approach that is QEMU-specific and strongly discourages third-party
> > device distribution or a muser-over-socket approach that offers a stable
> > API for VMM interoperability and third-party device distribution.
> The reason I dislike yet another offloading protocol (ie. there is
> vhost, there is vfio, and then there would be qdev-over-socket) is
> that we keep reinventing the wheel. I very much prefer picking
> something solid (eg. VFIO) and keep investing on it.

I like the idea of sticking close to VFIO too.  The first step is
figuring out whether VFIO can be mapped to a UNIX domain socket protocol
and many non-VFIO protocol messages are required.  Hopefully that extra
non-VFIO stuff isn't too large.

If implementations can use the kernel uapi vfio header files then we're
on track for compatibility with VFIO.

> > This is just a more elaborate explanation for the "the cat is out of the
> > bag" comments that have already been made on licensing.  Does anyone
> > still disagree or want to discuss further?
> > 
> > If there is agreement that a stable API is okay then I think the
> > practical way to do this is to first merge a cleaned-up version of
> > multi-process QEMU as an unstable experimental API.  Once it's being
> > tested and used we can write a protocol specification and publish it as
> > a stable interface when the spec has addressed most use cases.
> > 
> > Does this sound good?
> In that case, wouldn't it be preferable to revive our proposal from
> Edinburgh (KVM Forum 2018)? Our prototypes moved more of the Qemu VFIO
> code to "common" and added a "user" backend underneath it, similar to
> how vhost-user-scsi moved some of vhost-scsi to vhost-scsi-common and
> added vhost-user-scsi. It was centric on PCI, but it doesn't have to
> be. The other side can be implemented in libmuser for facilitating things.

That sounds good.


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