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

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

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).


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