On Mon, Oct 10, 2016 at 02:20:22AM +0300, Michael S. Tsirkin wrote:
> On Wed, Sep 28, 2016 at 10:28:48AM +0800, Yuanhan Liu wrote:
> > On Tue, Sep 27, 2016 at 10:56:40PM +0300, Michael S. Tsirkin wrote:
> > > On Tue, Sep 27, 2016 at 11:11:58AM +0800, Yuanhan Liu wrote:
> > > > On Mon, Sep 26, 2016 at 10:24:55PM +0300, Michael S. Tsirkin wrote:
> > > > > On Mon, Sep 26, 2016 at 11:01:58AM -0700, Stephen Hemminger wrote:
> > > > > > I assume that if using Version 1 that the bit will be ignored
> > > >
> > > > Yes, but I will just quote what you just said: what if the guest
> > > > virtio device is a legacy device? I also gave my reasons in another
> > > > email why I consistently set this flag:
> > > >
> > > > - we have to return all features we support to the guest.
> > > >
> > > > We don't know the guest is a modern or legacy device. That means
> > > > we should claim we support both: VERSION_1 and ANY_LAYOUT.
> > > >
> > > > Assume guest is a legacy device and we just set VERSION_1 (the
> > > > current
> > > > case), ANY_LAYOUT will never be negotiated.
> > > >
> > > > - I'm following the way Linux kernel takes: it also set both features.
> > > >
> > > > Maybe, we could unset ANY_LAYOUT when VERSION_1 is _negotiated_?
> > > >
> > > > The unset after negotiation I proposed turned out it won't work: the
> > > > feature is already negotiated; unsetting it only in vhost side doesn't
> > > > change anything. Besides, it may break the migration as Michael stated
> > > > below.
> > >
> > > I think the reverse. Teach vhost user that for future machine types
> > > only VERSION_1 implies ANY_LAYOUT.
>
> So I guess at this point, we can teach vhost-user in qemu
> that version 1 implies any_layout but only for machine types
> qemu 2.8 and up. It sets a bad precedent but oh well.
It should work.
--yliu
