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

[Michael S. Tsirkin]

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 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.

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.

-- 
MST