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

[Rusty Russell <rusty@au1.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> 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.

Well, the main issue is that it simply won't work.  Your guest without a
network or block device or console is usually a useless guest.

We should think through these issues for each transport, to make sure
they handle this case like I suggested for PCI, but I don't think it's
impossible.

Cheers,
Rusty.