Re: [PATCH v2] piix: fix regression during unplug in Xen HVM domUs

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 10 May 2023, Olaf Hering wrote:
> Wed, 10 May 2023 00:58:27 +0200 Olaf Hering <olaf@xxxxxxxxx>:
> 
> > In my debugging (with v8.0.0) it turned out the three pci_set_word
> > causes the domU to hang. In fact, it is just the last one:
> > 
> >    pci_set_byte(pci_conf + 0x20, 0x01);  /* BMIBA: 20-23h */
> > 
> > It changes the value from 0xc121 to 0x1.
> 
> If I disable just "pci_set_word(pci_conf + PCI_COMMAND, 0x0000);" it works as 
> well.
> It changes the value from 0x5 to 0.
> 
> In general I feel it is wrong to fiddle with PCI from the host side.
> This is most likely not the intention of the Xen unplug protocol.
> I'm sure the guest does not expect such changes under the hood.
> It happens to work by luck with pvops kernels because their PCI discovery
> is done after the unplug.
> 
> So, what do we do here to get this off the table?

I don't have a concrete suggestion because I don't understand the root
cause of the issue. Looking back at Paolo's reply from 2021

https://marc.info/?l=xen-devel&m=161669099305992&w=2

I think he was right. We can either fix the root cause of the issue or
avoid calling qdev_reset_all on unplug. I am OK with either one.