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

[Michael S. Tsirkin]

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?

Also why is there SET_CONFIG? Does not look like blk uses it.

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.


-- 
MST