[Qemu-devel] Re: [PATCH] PCI: Bus number from the bridge, not the device

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 2010-11-08 at 13:22 +0200, Michael S. Tsirkin wrote:
> On Mon, Oct 04, 2010 at 03:53:11PM -0600, Alex Williamson wrote:
> > pcibus_dev_print() was erroneously retrieving the device bus
> > number from the secondary bus number offset of the device
> > instead of the bridge above the device.  This ends of landing
> > in the 2nd byte of the 3rd BAR for devices, which thankfully
> > is usually zero.  pcibus_get_dev_path() copied this code,
> > inheriting the same bug.  pcibus_get_dev_path() is used for
> > ramblock naming, so changing it can effect migration.  However,
> > I've only seen this byte be non-zero for an assigned device,
> > which can't migrate anyway, so hopefully we won't run into
> > any issues.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> 
> Good catch. Applied.
> I don't really see why do we put the dev path
> in the bus object: why not let device supply its name?

Because the device name is not unique.  This came about from the
discussion about how to create a canonical device path that Gleb and
Markus are again trying to hash out.  If we go up to the bus and get the
bus address, we have a VM unique name.  Unfortunately, it's difficult to
define what the bus should print in all cases (ISA), but since they
don't do hotplug and typically don't allocate ramblocks, we can mostly
ignore it for this use case.

> And I think this will affect nested bridges. However they are currently
> broken anyway: we really must convert to topological names as bus number
> is guest-assigned - they don't have to be unique, even.

Yes, nested bridges are a problem.  How can the seg/bus/devfn not be
unique?

> What does fixing this involve? Just changing pcibus_get_dev_path?

How do you plan to fix it?  Don't forget that migration depends on these
names, so some kind of compatibility layer would be required.  Thanks,

Alex

> > ---
> > 
> >  hw/pci.c |    5 ++---
> >  1 files changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/pci.c b/hw/pci.c
> > index 6d0934d..15416dd 100644
> > --- a/hw/pci.c
> > +++ b/hw/pci.c
> > @@ -1940,8 +1940,7 @@ static void pcibus_dev_print(Monitor *mon, DeviceState *dev, int indent)
> >  
> >      monitor_printf(mon, "%*sclass %s, addr %02x:%02x.%x, "
> >                     "pci id %04x:%04x (sub %04x:%04x)\n",
> > -                   indent, "", ctxt,
> > -                   d->config[PCI_SECONDARY_BUS],
> > +                   indent, "", ctxt, pci_bus_num(d->bus),
> >                     PCI_SLOT(d->devfn), PCI_FUNC(d->devfn),
> >                     pci_get_word(d->config + PCI_VENDOR_ID),
> >                     pci_get_word(d->config + PCI_DEVICE_ID),
> > @@ -1965,7 +1964,7 @@ static char *pcibus_get_dev_path(DeviceState *dev)
> >      char path[16];
> >  
> >      snprintf(path, sizeof(path), "%04x:%02x:%02x.%x",
> > -             pci_find_domain(d->bus), d->config[PCI_SECONDARY_BUS],
> > +             pci_find_domain(d->bus), pci_bus_num(d->bus),
> >               PCI_SLOT(d->devfn), PCI_FUNC(d->devfn));
> >  
> >      return strdup(path);
> >