Re: [PATCH 1/4] s390x/pci: use a reserved ID for the default PCI group

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 2 Dec 2021 12:11:38 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> > 
> > What happens if we migrate a VM from old to new QEMU? Won't the guest be
> > able to observe the change?
> >   
> 
> Yes, technically --  But # itself is not really all that important, it 
> is provided from CLP Q PCI FN to be subsequently used as input into Q 
> PCI FNGRP -- With the fundamental notion being that all functions that 
> share the same group # share the same group CLP info.  Whether the 
> number is, say, 1 or 5 doesn't matter so much.
> 
> However..  0xF0 and greater are the only values reserved for hypervisor 
> use.  By using 0x20 we run the risk of accidentally conflating simulated 
> devices and real hardware, hence the desire to change it.
> 
> Is your concern about a migrated guest with a virtio device trying to do 
> a CLP QUERY PCI FNGRP using 0x20 on a new QEMU?  I suppose we could 
> modify 'clp_service_call, case CLP_QUERY_PCI_FNGRP' to silently catch 
> simulated devices trying to use something other than the default group, 
> e.g.:
> 
> if ((pbdev->fh & FH_SHM_EMUL) &&
>      (pbdev->zpci_fn.pfgid != ZPCI_DEFAULT_FN_GRP)) {
>          /* Simulated device MUST have default group */
> 	pbdev->zpci_fn.pfgid = ZPCI_DEFAULT_FN_GRP;
> 	group = s390_group_find(ZPCI_DEFAULT_FN_GRP);
> }
> 
> What do you think?

Another option, and in my opinion the cleaner one would be to tie this
change to a new machine version. That is if a post-change qemu is used
in compatibility mode, we would still have the old behavior.

What do you think?

Regards,
Halil