Re: [Qemu-devel] [PATCH v5 1/4] vhost-user: add new vhost user messages to support virtio config space

[Michael S. Tsirkin]

On Tue, Nov 21, 2017 at 01:16:12AM +0100, Paolo Bonzini wrote:
> On 20/11/2017 21:44, Michael S. Tsirkin wrote:
> > Live migrations is supposed to be migrating guest writeable state too.
> > If you mean migrating RO fields like size, then
> > I don't think it's a good idea to reuse SET_CONFIG for that.
> > SET_CONFIG should obey exactly the virtio semantics.
> > 
> > And I agree, it should say that slave must treat it as a write,
> > and get config as a read according to virtio semantics.
> > 
> > If someone needs to pass configuration from qemu to
> > slave, let's add specific messages with precisely defined semantics.
> 
> Fair enough, but I'd add nevertheless a 32-bit flags field to both
> GET_CONFIG and SET_CONFIG, and document that the slave MUST check that
> it is zero and otherwise fail.
> 
> Paolo

We generally just use protocol feature bits for extensions.
But I have no problem with reserved padding if desired
which seems to be what's described here.

-- 
MST