Re: Config space access to Mediatek MT7922 doesn't work after device reset in Xen PV dom0 (regression, Linux 6.12)

[Bjorn Helgaas <helgaas@xxxxxxxxxx>]

On Wed, Jan 29, 2025 at 03:10:49AM +0100, Marek Marczykowski-Górecki wrote:
> On Tue, Jan 28, 2025 at 07:15:26PM -0600, Bjorn Helgaas wrote:
> > On Fri, Jan 17, 2025 at 01:05:30PM +0100, Marek Marczykowski-Górecki wrote:
> > > After updating PV dom0 to Linux 6.12, The Mediatek MT7922 device reports
> > > all 0xff when accessing its config space. This happens only after device
> > > reset (which is also triggered when binding the device to the
> > > xen-pciback driver).
> > 
> > Thanks for the report and for all the debugging you've already done!
> > 
> > > Reproducer:
> > > 
> > >     # lspci -xs 01:00.0
> > >     01:00.0 Network controller: MEDIATEK Corp. MT7922 802.11ax PCI 
> > > Express Wireless Network Adapter
> > >     00: c3 14 16 06 00 00 10 00 00 00 80 02 10 00 00 00
> > >     ...
> > >     # echo 1 > /sys/bus/pci/devices/0000:01:00.0/reset
> > >     # lspci -xs 01:00.0
> > >     01:00.0 Network controller: MEDIATEK Corp. MT7922 802.11ax PCI 
> > > Express Wireless Network Adapter
> > >     00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > >
> > > The same operation done on Linux 6.12 running without Xen works fine.
> > > 
> > > git bisect points at:
> > > 
> > >     commit d591f6804e7e1310881c9224d72247a2b65039af
> > >     Author: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > >     Date:   Tue Aug 27 18:48:46 2024 -0500
> > > 
> > >     PCI: Wait for device readiness with Configuration RRS
> > > 
> > > part of that commit:
> > > @@ -1311,9 +1320,15 @@ static int pci_dev_wait(struct pci_dev *dev, char 
> > > *reset_type, int timeout)
> > >                         return -ENOTTY;
> > >                 }
> > >  
> > > -               pci_read_config_dword(dev, PCI_COMMAND, &id);
> > > -               if (!PCI_POSSIBLE_ERROR(id))
> > > -                       break;
> > > +               if (root && root->config_crs_sv) {
> > > +                       pci_read_config_dword(dev, PCI_VENDOR_ID, &id);
> > > +                       if (!pci_bus_crs_vendor_id(id))
> > > +                               break;
> > > +               } else {
> > > +                       pci_read_config_dword(dev, PCI_COMMAND, &id);
> > > +                       if (!PCI_POSSIBLE_ERROR(id))
> > > +                               break;
> > > +               }
> > >  
> > >     
> > > Adding some debugging, the PCI_VENDOR_ID read in pci_dev_wait() returns
> > > initially 0xffffffff. If I extend the condition with
> > > "&& !PCI_POSSIBLE_ERROR(id)", then the issue disappear. But reading the
> > > patch description, it would break VF.
> > > I'm not sure where the issue is, but given it breaks only when running
> > > with Xen, I guess something is wrong with "Configuration RRS Software
> > > Visibility" in that case.
> > 
> > I'm missing something.  If you get 0xffffffff, that is not the 0x0001
> > Vendor ID, so pci_dev_wait() should exit immediately.  
> 
> I'm not sure what is going on there either, but my _guess_ is that the
> loop exits too early due to the above. And it makes some further actions
> to fail.

Seems like a good guess worth investigating.  Maybe log all config
accesses to this device after the FLR and see what we're doing?