Re: [Qemu-devel] [PATCH v4 1/4] vhost-user: add new vhost user messages to support virtio config space

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 20, 2017 at 01:55:20AM +0000, Liu, Changpeng wrote:
> 
> 
> > -----Original Message-----
> > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > Sent: Friday, October 20, 2017 8:28 AM
> > To: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Liu, Changpeng <changpeng.liu@intel.com>; qemu-devel@nongnu.org;
> > stefanha@gmail.com; marcandre.lureau@redhat.com; felipe@nutanix.com; Harris,
> > James R <james.r.harris@intel.com>
> > Subject: Re: [PATCH v4 1/4] vhost-user: add new vhost user messages to support
> > virtio config space
> > 
> > On Thu, Oct 19, 2017 at 11:04:48PM +0200, Paolo Bonzini wrote:
> > > On 19/10/2017 19:43, Michael S. Tsirkin wrote:
> > > > On Thu, Oct 19, 2017 at 05:43:18PM +0200, Paolo Bonzini wrote:
> > > >> On 19/10/2017 17:39, Michael S. Tsirkin wrote:
> > > >>>> Add VHOST_USER_GET_CONFIG/VHOST_USER_SET_CONFIG messages
> > which can be
> > > >>>> used for live migration of vhost user devices, also vhost user devices
> > > >>>> can benefit from the messages to get/set virtio config space from/to the
> > > >>>> I/O target. For the purpose to support virtio config space change,
> > > >>>> VHOST_USER_SET_CONFIG_FD message is added as the event notifier
> > > >>>> in case virtio config space change in the I/O target.
> > > >>>>
> > > >>>> Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
> > > >>> I don't much like it that config is getting passed through.
> > > >>>
> > > >>> IMO this makes managing things harder not easier.
> > > >>>
> > > >>> How about specific messages about specific parts of
> > > >>> config space that you want to get from the backend?
> > > >>
> > > >> In the case of virtio-blk that would be all of it.  Do you have a case
> > > >> in mind where some part of the configuration space is owned by QEMU?
> > > >>
> > > >> Paolo
> > > >
> > > > Yes. seg_max
> > >
> > > The seg_max limit is established by whoever reads buffers from the vring
> > > and passes them down to the lower layer.  For vhost-blk that's the
> > > device server, not QEMU.
> > >
> > > Paolo
> > 
> > Good point. How about num_queues though?
> num_queues  is part of virtio_blk config, vhost-user slave can set it, 
> and Qemu driver can rewrite it if user want less IO queues.

Fundamentally QEMU needs to support this # of queues for this
device.

So whenever QEMU doesn't always expose config space as-is,
need to document the exact semantics.

Also, does backend need to know?


> > 
> > Also why is there SET_CONFIG? Does not look like blk uses it.
> Only one possible usage when disable write cache to vhost-user slave device.

Again need to add documentation what can be written.


> > 
> > And I wonder how do we do it for other devices.
> > 
> > E.g. for net there's a bit in the middle of the
> > config field that deals with migration.
> Well, I'm okay to make those messages only valid for virtio block device, because it's enough
> for virtio block to be started with vhost-user slave target.

OK but I'd rather make them at least somewhat generic so we can reuse
them down the road.  It looks like adding offset/size pair would solve
most of the issues. Thoughts?

> > 
> > 
> > --
> > MST