Re: [virtio-dev] [PATCH] transitional issues: add new IDs for all devices

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

On Wed, Oct 02, 2013 at 08:20:33AM +0300, Michael S. Tsirkin wrote:
> On Wed, Oct 02, 2013 at 10:02:28AM +0930, Rusty Russell wrote:
> > "Michael S. Tsirkin" <mst@redhat.com> writes:
> > > On Thu, Sep 26, 2013 at 11:35:53AM +0930, Rusty Russell wrote:
> > >> How about a PCI note something like:
> > >> 
> > >>         If a device does not support legacy mode, and is on a platform
> > >>         where a legacy device with the same ID has previously existed,
> > >>         it MUST take the following steps to fail gracefully when a
> > >>         legacy driver attempts to drive it:
> > >> 
> > >>         1) Present no I/O BAR, and no BAR 0, OR
> > >>         2) Respond to a single-byte zero write to offset 18 of any I/O
> > >>            BAR or BAR0 by presenting zeroes on every BAR and ignoring
> > >>            writes.
> > >> 
> > >> This means it presents 0 virtqueues; at the very least, it makes the
> > >> device harmless.
> > >> 
> > >> Note that I didn't specify that it should only be the first write: think
> > >> of the kexec case, where an OS crash causes an older OS to come in and
> > >> try to reset the device...
> > >> 
> > >> Would that work?
> > >> Rusty.
> > >
> > > So I guess we can also suggest revision id > 0 for pci
> > > and > 1 for mmio for non transitional devices.
> > 
> > Only if mmio changes its layout in an incompatible way: that's not
> > confimed yet.  Otherwise there's no reason to worry about older drivers
> > as the device can fail gracefully when feature 32 isn't acknowledged.
> > 
> > > At least with recent windows drivers, this will be enough
> > > to not make them bind, and it will also work for
> > > linux drivers.
> > >
> > > Question: what to do if we want do create a non transitional
> > > device on ccw?
> > 
> > I think they're happy with their current layout, but I leave this to
> > Cornelia (CC'd, but she's still away).
> > 
> > Cheers,
> > Rusty.
> 
> Well this means they aren't interested in addressing
> https://tools.oasis-open.org/issues/browse/VIRTIO-40
> ?
> Also, endian-ness change if it goes through is enough to
> break drivers in weird ways, isn't it?

Also, changes like driver initialization change also
make for a bunch of interesting bugs.
If we let legacy drivers bind to non transitional
devices, that just makes for a bunch of weird issues
to consider.

> 
> -- 
> MST