Re: [RFC 2/8] s390x/pci: MSI-X isn't strictly required for passthrough

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 10 Dec 2020 10:13:29 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

> On 12/10/20 5:28 AM, Cornelia Huck wrote:
> > On Wed,  9 Dec 2020 15:34:20 -0500
> > Matthew Rosato <mjrosato@linux.ibm.com> wrote:
> >   
> >> s390 PCI currently disallows PCI devices without the MSI-X capability.
> >> However, this fence doesn't make sense for passthrough devices.  Move
> >> the check to only fence emulated devices (e.g., virtio).
> >>
> >> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> >> Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
> >> ---
> >>   hw/s390x/s390-pci-bus.c | 14 ++++++++------
> >>   1 file changed, 8 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
> >> index 05f7460..afad048 100644
> >> --- a/hw/s390x/s390-pci-bus.c
> >> +++ b/hw/s390x/s390-pci-bus.c
> >> @@ -1028,12 +1028,12 @@ static void s390_pcihost_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >>               s390_pci_get_clp_info(pbdev);
> >>           } else {
> >>               pbdev->fh |= FH_SHM_EMUL;
> >> -        }
> >>   
> >> -        if (s390_pci_msix_init(pbdev)) {
> >> -            error_setg(errp, "MSI-X support is mandatory "
> >> -                       "in the S390 architecture");
> >> -            return;
> >> +            if (s390_pci_msix_init(pbdev)) {
> >> +                error_setg(errp, "MSI-X support is mandatory "
> >> +                           "in the S390 architecture");
> >> +                return;
> >> +            }
> >>           }
> >>   
> >>           if (dev->hotplugged) {
> >> @@ -1073,7 +1073,9 @@ static void s390_pcihost_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >>           devfn = pci_dev->devfn;
> >>           qdev_unrealize(dev);
> >>   
> >> -        s390_pci_msix_free(pbdev);
> >> +        if (pbdev->fh & FH_SHM_EMUL) {
> >> +            s390_pci_msix_free(pbdev);
> >> +        }
> >>           s390_pci_iommu_free(s, bus, devfn);
> >>           pbdev->pdev = NULL;
> >>           pbdev->state = ZPCI_FS_RESERVED;  
> > 
> > Remind me: Wasn't it only msi that was strictly required (i.e., not msi-x?)
> > 
> > Can we generally relax this requirement, possibly with some changes in
> > the adapter interrupt mapping? I might misremember, though.
> >   
> 
> Yes, but even so our current emulation support only sets up for MSI-X, 
> it does not have an msi_init() equivalent.  I do believe that this 
> requirement can be relaxed at some point for the emulation support as 
> well, but the focus on this set was to at least stop fencing passthrough 
> for no reason.

Looking back to when this was introduced, I see that 857cc71985dc
("s390x/pci: merge msix init functions") actually makes this mandatory
and states that nothing changes for passthrough. Has anything changed
regarding msi-x in the architecture in the meantime?