Re: [Qemu-devel] [PATCH] add pci-bridge-seat

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

On Tue, Mar 17, 2015 at 08:16:45AM +0100, Gerd Hoffmann wrote:
> On Mo, 2015-03-16 at 13:20 +0100, Michael S. Tsirkin wrote:
> > On Mon, Mar 16, 2015 at 11:36:43AM +0100, Gerd Hoffmann wrote:
> > > Simplifies multiseat configuration, see
> > > docs/multiseat.txt update for details.
> > > 
> > > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> > > ---
> > >  docs/multiseat.txt             | 19 +++++++++++++++++++
> > >  docs/specs/pci-ids.txt         |  1 +
> > >  hw/pci-bridge/pci_bridge_dev.c | 25 ++++++++++++++++++++++++-
> > >  include/hw/pci/pci.h           |  1 +
> > >  4 files changed, 45 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/docs/multiseat.txt b/docs/multiseat.txt
> > > index b963665..814496e 100644
> > > --- a/docs/multiseat.txt
> > > +++ b/docs/multiseat.txt
> > > @@ -106,6 +106,25 @@ the devices attached to the seat.
> > >  Background info is here:
> > >    http://www.freedesktop.org/wiki/Software/systemd/multiseat/
> > >  
> > > +
> > > +guest side with pci-bridge-seat
> > > +-------------------------------
> > > +
> > > +Qemu version FIXME and newer has a new pci-bridge-seat device which
> > > +can be used instead of pci-bridge.  Just swap the device name in the
> > > +qemu command line above.  The only difference between the two devices
> > > +is the pci id.  We can match the pci id instead of the device path
> > > +with a nice generic rule now, which simplifies the guest
> > > +configuration:
> > > +
> > > +    [root@fedora ~]# cat /etc/udev/rules.d/70-qemu-pci-bridge-seat.rules
> > > +    SUBSYSTEM=="pci", ATTR{vendor}=="0x1b36", ATTR{device}=="0x000a", \
> > > +            TAG+="seat", ENV{ID_AUTOSEAT}="1"
> > > +
> > > +Patch with this rule will be submitted to upstream udev/systemd, so
> > > +long-term, when systemd with this lands in distros, things will work
> > > +just fine without any manual guest configuration.
> > > +
> > >  Enjoy!
> > >  
> > 
> > I'm confused. What's wrong with using the regular bridge,
> > and ATTR{vendor}=="0x1b36", ATTR{device}=="0x0001"?
> 
> It works just fine, thats why the new device is identical except for the
> id.
> 
> The whole point is to simplify guest-side configuration, by passing one
> bit of extra information to the guest:  Whenever the pci bridge with the
> devices below it should be considered a new seat (-device
> pci-bridge-seat) or not (-device pci-bridge).
> 
> cheers,
>   Gerd
> 

Could you explain a bit more please? What does it mean that
a bridge is a new seat? How is this used?