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

[Paolo Bonzini]

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