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

[Liu, Changpeng]

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:address@hidden
> Sent: Friday, October 20, 2017 8:28 AM
> To: Paolo Bonzini <address@hidden>
> Cc: Liu, Changpeng <address@hidden>; address@hidden;
> address@hidden; address@hidden; address@hidden; Harris,
> James R <address@hidden>
> 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 <address@hidden>
> > >>> 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.
> 
> 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.
> 
> 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.
> 
> 
> --
> MST