Re: [Qemu-devel] [PATCH 2/5] virtio-blk: disable scsi passthrough for 1.0 device

[Cornelia Huck]

On Wed, 15 Jul 2015 13:59:00 +0300
"Michael S. Tsirkin" <address@hidden> wrote:

> On Tue, Jul 14, 2015 at 07:43:44PM +0200, Cornelia Huck wrote:
> > > Yes, and that's because as written, transitional devices must set
> > > ANY_LAYOUT, but that's incompatible with scsi.
> > 
> > Hm, I had a patch before that dynamically allowed different feature
> > sets for legacy or modern, not only a subset. Probably won't apply
> > anymore, but I'd like to able to do the following:
> > 
> > - driver reads features without negotiating a revision: driver is
> >   legacy, offer legacy bits
> > - driver negotiates revision 0: dito
> > - driver negotiates revision >= 1: driver is modern, offer modern bits
> > 
> > That way we could offer SCSI and !ANY_LAYOUT (if scsi is enabled) in the
> > first two cases, and a new qemu could still offer scsi to old guests.
> > 
> > Would it be worth pursuing that idea?
> 
> Frankly, I don't think so: I don't see why it makes sense
> to expose more features on the legacy interface than
> on the modern one. Imagine updating drivers to fix a bug
> and losing some features. How does this make sense?

I don't think one should be a strict subset of the other. But I think
we don't want to withdraw features from legacy guests on qemu updates
either?

> 
> I think the virtio TC's assumption was that the scsi passthrough was a
> bad idea, so in QEMU we only keep it around for legacy devices to avoid
> regressions.

I'm not opposing this :)

> 
> If you disagree and think transitional devices need the SCSI feature,
> either try to convince pbonzini or rewrite the spec youself
> to support it in the virtio 1 mode.

This seems to boil down to the different meaning of "transitional" for
ccw and pci, see the other thread.