Re: [Qemu-devel] [PATCH 4/4] virtio-ccw: enable virtio-1

[Cornelia Huck]

On Thu, 10 Sep 2015 12:22:42 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Thu, Sep 10, 2015 at 11:11:20AM +0200, Cornelia Huck wrote:
> > On Thu, 10 Sep 2015 12:07:18 +0300
> > "Michael S. Tsirkin" <address@hidden> wrote:
> > 
> > > On Fri, Sep 04, 2015 at 10:54:29AM +0200, Cornelia Huck wrote:
> > > > Let's enable revision 1 for virtio-ccw devices. We can always offer
> > > > VERSION_1 as drivers in legacy mode won't be able to see it anyway.
> > > > 
> > > > We have to introduce a way to set a lower maximum revision for a device
> > > > to accommodate the following cases:
> > > > - compat machines (to enforce legacy only)
> > > 
> > > But you don't actually set this for any compat machines.
> > > If you don't, this seems a bit pointless.
> > 
> > Huh? The first hunk of this patch does this.
> 
> Sure. Donnu how I could miss it, sorry.
> 
> > > 
> > > > - virtio-blk with scsi support (version 1 + scsi is fenced by common
> > > >   code, with a user-configured max revision of 0 we can allow scsi
> > > >   via not offering VERSION_1)
> > > 
> > > For this use, for pci users need to do disable_modern=true.
> > > I find it unfortunate that for ccw one needs to do max_revision=0.
> > 
> > I don't like the pci concept: much too coarse-grained and not very
> > future proof.
> > 
> > > 
> > > Revision numbers generally are a ccw specific concept. I'm not sure it
> > > is wise to expose it to users.
> > 
> > What is wrong about exposing transport-specific concepts?
> 
> Nothing. Go ahead and expose as much of the low level as
> makes sense.
> 
> But it would be nice if there was also a portable way for people
> that just want "virtio" and don't care about the low level details
> of which transport it is.

Proxy devices already look different depending on what transport you
use, so I don't think it really matters.

(And for most users, I'd expect they let a management layer take care
of it anyway.)

> 
> OTOH a conservative estimate of the # of people that will want
> to play with this is pretty close to 0, so maybe it does not matter
> much.

The scsi vs. virtio-1 on ccw handling? Not very many (if anyone), yes.

> 
> > > 
> > > > Signed-off-by: Cornelia Huck <address@hidden>
> > > > ---
> > > >  hw/s390x/s390-virtio-ccw.c |  4 ++++
> > > >  hw/s390x/virtio-ccw.c      | 30 ++++++++++++++++++++++++++++--
> > > >  hw/s390x/virtio-ccw.h      |  6 ++++--
> > > >  3 files changed, 36 insertions(+), 4 deletions(-)