Re: [virtio] Re: [virtio-comment] [PATCH RFC] virtio-pci: new configuration layout

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Mon, 02 Sep 2013 10:56:20 +0930
Rusty Russell <rusty@au1.ibm.com> wrote:

> Cornelia Huck <cornelia.huck@de.ibm.com> writes:
> > Some remarks from my side...
> >
> > On Tue, 27 Aug 2013 10:38:59 +0300
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >
> >> On Tue, Aug 27, 2013 at 04:24:47PM +0930, Rusty Russell wrote:
> >> > "Michael S. Tsirkin" <mst@redhat.com> writes:
> >> > > This is the new configuration layout.
> >> > >
> >> > > Notes:
> >> > > - Everything is LE
> >> > > - There's a feature bit that means spec 1.0 compliant.
> >> > > - Both devices and drivers can either require the 1.0 interface
> >> > >   or try to include compatibility support. The spec isn't forcing
> >> > >   this decision.
> >> > 
> >> > Hmm, this kind includes other changes already proposed, like the LE
> >> > change and the framing change.  I think this conceptually splits nicely:
> >> > 
> >> > 1) Feature bit 32 proposal.
> >> > 2) Endian change.
> >> > 3) Framing change.
> >> > 4) PCI layout change.
> >> 
> >> Right - they are mostly in different parts of the document.
> >> I put it all together so it's easy to see how we intend to
> >> handle the transition.
> >> So is everyone OK with keeping this in a single patch?
> >
> > The new feature bit is supposed to cover all of this, right? Then this
> > should be one patch.
> 
> Well, the new feature would be "1.0 compliant" which will cover much
> more than this.  We'll have a whole document on the differences between
> legacy and 1.0, in fact.
> 
> >> > > - I kept documentation of the legacy interface around, and added notes
> >> > >   on transition inline. They are in separate sections each clearly marked
> >> > >   "Legacy Interface" so we'll be able to separate them out
> >> > >   from the final document as necessary - for now I think it's easier
> >> > >   to keep it all together.
> >> > 
> >> > Good thinking: most of us know the current spec so it's definitely
> >> > clearer.  And makes sure we're thinking about the transition.
> >> > 
> >> > > Only virtio PCI has been converted.
> >> > > Let's discuss this on the meeting tonight, once we figure out PCI
> >> > > we can do something similar for MMIO and CCW.
> >> > 
> >> > > @@ -137,6 +139,11 @@ Feature bits are allocated as follows:
> >> > >    24 to 31: Feature bits reserved for extensions to the queue and 
> >> > >    feature negotiation mechanisms
> >> > >  
> >> > > +  32: Feature bit must be set for any device compliant with this
> >> > > +  revision of the specification, and acknowledged by all device drivers.
> >
> > Would it make sense to have a bit 33 "rings big endian" whose validity
> > depends on bit 32 set? This would make it possible for ccw to keep its
> > current endianness.
> 
> Very awkward.  Our experience with bi-endian devices on PowerPC suggests
> it's far better to not negotiate endian.  It also avoids a branch
> everywhere in the driver and host.

Ok, so let's skip this feature bit and make virtio-ccw big endian for
the ring?

> 
> >> > > +Legacy Interface
> >> > > +	is an interface specified by an earlier draft of this specification
> >> > > +        (up to 0.9.X)
> >> > > +Legacy Device
> >> > > +	is a device implemented before this specification was released,
> >> > > +        and implementing a legacy interface on the host side
> >> > > +Legacy Driver
> >> > > +	is a driver implemented before this specification was released,
> >> > > +        and implementing a legacy interface on the guest side
> >> > > +
> >> > > +to simplify transition from these earlier draft interfaces,
> >> > > +it is possible to implement
> >> > > +
> >> > > +Transitional Device
> >> > > +	a device supporting both drivers conforming to this
> >> > > +        specification, and legacy drivers
> >> > > +
> >> > > +Transitional Driver
> >> > > +	a driver supporting both devices conforming to this
> >> > > +	specification, and legacy devices
> >
> > What happens to legacy devices in the future? Current implementers
> > will obviously expose legacy devices, which means future drivers need
> > to be transitional or they won't work with what is currently out there.
> > Will legacy stay around (for the forseeable furture)? Will legacy
> > devices still be considered standard compliant (as in "compliant to the
> > legacy standard")?
> 
> Legacy devices will not be spec compliant, though we will map out the
> differences for implementors and to make sure that transitional devices
> & drivers are still possible.  This legacy part will eventually be a
> separate document.
> 
> I expect them legacy to be firmly deprecated within a few years: new
> features will probably only be offered for standard devices, and for PCI
> there are some nice proposals in the standard which make life easier,
> eg. for bootloaders.

Is there any reason the legacy version can't stay in, with a big fat
warning "new devices are not supposed to implement this"?

Another thing we need to keep in mind is that switching a device driver
to support a new version of a device is not the way it works on every
system. On s390, there's generally no device driver as such - Linux is
the oddball there.

> 
> >> > > +
> >> > > +Device and driver that require support for revision 1.0 or newer of
> >> > > +the specification to function, are called non-transitional device and driver,
> >> > > +respectively.
> >> > > +
> >> > > +Transitional Drivers can detect Legacy Devices by detecting that
> >> > > +Feature bit 32 is not offered.
> >> > > +Transitional devices can detect Legacy drivers by detecting that
> >> > > +Feature bit 32 has not been acknowledged by driver.
> >
> > Will we use new feature bits for new, incompatible revisions? Or will
> > we try to stay backwards compatible?
> 
> My intent is to stay backwards compatible, using feature bits.  At some
> future point we may deprecate older features I suppose.

ok

> 
> >> > > +2.1.4.2.1 Legacy Interfaces: A Note on Message Framing
> >> > > +-----------------------
> >> > > +Regrettably, initial driver implementations used simple layouts, and
> >> > > +devices came to rely on it, despite this specification wording[10]. It
> >> > > +is thus recommended that when using legacy interfaces,
> >> > > +drivers should be conservative in their assumptions,
> >> > > +unless the VIRTIO_F_ANY_LAYOUT feature is accepted.
> >
> > So ANY_LAYOUT and feature bit 32 are mutually exclusive?
> 
> I was thinking feature 32 would imply ANY_LAYOUT; there's a feature
> request for that:
>         https://tools.oasis-open.org/issues/browse/VIRTIO-10
> 
> Since there seems to be consensus that we remove the SCSI commands from
> virtio-blk, it seems sensible to do that (which is a blocker for
> ANY_LAYOUT) and then remove ANY_LAYOUT from the non-legacy wording and
> make it clear that we're always independent of framing.

I was a bit confused by the diff, but that sounds sensible.

> 
> >> Also Revision ID is only 8 bit - it's designed for hardware where
> >> making a new revision is expensive. In software we'll run out of that
> >> eventually.
> >
> > So Revision ID is a PCI-specific thing, right? Not all transports will
> > necessarily have something equivalent, so they would need to depend on
> > the feature bit.
> 
> Yes.
> 
> Cheers,
> Rusty.

Cornelia