Re: [Qemu-devel] [PATCH] Correctly assign PCI domain numbers

[Michael S. Tsirkin]

On Tue, Aug 02, 2011 at 12:15:22AM +1000, David Gibson wrote:
> On Mon, Aug 01, 2011 at 05:03:18PM +0300, Michael S. Tsirkin wrote:
> > On Mon, Aug 01, 2011 at 11:33:37PM +1000, David Gibson wrote:
> > > On Mon, Aug 01, 2011 at 01:10:38PM +0300, Michael S. Tsirkin wrote:
> > > > On Mon, Aug 01, 2011 at 04:51:02PM +1000, David Gibson wrote:
> > > > > qemu already almost supports PCI domains; that is, several entirely
> > > > > independent PCI host bridges on the same machine.  However, a bug in
> > > > > pci_bus_new_inplace() means that every host bridge gets assigned 
> > > > > domain
> > > > > number zero and so can't be properly distinguished.  This patch fixes 
> > > > > the
> > > > > bug, giving each new host bridge a new domain number.
> > > > > 
> > > > > Signed-off-by: David Gibson <address@hidden>
> > > > 
> > > > OK, but I'd like to see the whole picture.
> > > > How does the guest detect multiple domains,
> > > > and how does it access them?
> > > 
> > > For the pseries machine, which is what I'm concerned with, each host
> > > bridge is advertised through the device tree passed to the guest.
> > 
> > Could you explain please?
> > What generates the device tree and passes it to the guest?
> 
> In the case of the pseries machine, it is generated from hw/spapr.c
> and loaded into memory for use by the firmware and/or the kernel.
> 
> > > That gives the necessary handles and addresses for accesing config
> > > space and memory and IO windows for each host bridge.
> > 
> > I see. I think maybe a global counter in the common code
> > is not exactly the best solution in the general case.
> 
> Well, which general case do you have in mind. Since by definition,
> PCI domains are entirely independent from each other, domain numbers
> are essentially arbitrary as long as they're unique - simply a
> convention which makes it easier to describe which host bridge devices
> belong on.  I don't see an obvious approach which is better than a
> global counter, or least not one that doesn't involve a significant
> rewrite of the PCI subsystem.


OK, let's make sure I understand. On your system
'domain numbers' are completely invisible to the
guest, right? You only need them to address
devices on qemu monitor ...

For that, I'm trying to move away from using
a domain number.
Would it be possible to simply give  bus an id,
and use bus=<id> instead?


BTW, how does a linux guest number domains?
Would it make sense to match that?


-- 
MST