Re: [virtio] [OASIS Issue Tracker] Created: (VIRTIO-35) race condition with multi-dword config accesses

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 09, 2013 at 03:33:33PM +0100, Pawel Moll wrote:
> On Tue, 2013-10-08 at 21:59 +0100, Michael S. Tsirkin wrote:
> > > > MUST always look at the
> > > > VIRTIO_1 feature bit to check whether device supports the new interface
> > > > (as opposed to checking the revision).
> > > 
> > > The version register has one intention - make the changes of the control
> > > registers layout (except for the first two of them :-) easy. Sounds like
> > > this is where we are. No if, buts or guessing - different register
> > > layout, different version. So that's the transport.
> > > 
> > > The feature bit is still required for the functional drivers (block,
> > > net, scsi etc). Unless I don't understand the idea :-)
> > > 
> > > It seems that it would be possible to have a legacy functional device
> > > with new MMIO control register layout. I'm not sure this would be
> > > useful, but it should be possible.
> > 
> > I'm sorry I don't understand. We have a set of terms
> > defined in the spec now- transitional non transitinal legacy.
> > Can we frame discussion in these terms please?
> > Changing version makes it a non transitional device.
> > I am asking for ability to make transitinal
> > devices as well, this means that
> > - devices must have ability to have version 1
> > - drivers must not use version to detect legacy interface
> 
> You forgot to define what do "device" and "driver" mean then :-P
> 
> I look at the problem as being divided into the transport interface
> (driven in Linux by platform_driver virtio_mmio_driver) and the virtio
> device (driven by struct virtio_driver virtio_blk). Is there any problem
> with this point of view?
> 
> Now, the virtio_blk driver can be legacy, transitional or
> non-transitional. I have no problem with that. MMIO transport is
> completely transparent in terms of features, including
> VIRTIO_F_VERSION_1.
> 
> At the same time virtio_mmio *driver* will be able to drive only version
> 1 of the transport, only version 2 of the transport or both versions of
> the transport. The host-side transport implementation will have register
> layout of version 1 *or* 2 and will *not* change behaviour in runtime
> depending on VIRTIO_F_VERSION_1. Is there any problem with this?
> If you
> prefer I can call version 1 "legacy", and version 2 "non-transitional".
> I have no problem with this.
> 
> I believe that with this approach, everything should Just Work (TM).
> 
> Paweł

Fine, but incomplete.

What I am saying is that version field must not be the version of
the transport.
It must only be there to declare that device is non transitional.

The virtio_mmio driver must not look at
the version field to detect whether device supports virtio 1.0.
Instead it must only look at whether device exposes VIRTIO_F_VERSION_1.

I believe with this approach,  everything should Just Work (TM)
*and* if someone wants to implement a transitional device
after all, one will be able to .

-- 
MST