Re: [PATCH v10 06/12] pcie_sriov: Reuse SR-IOV VF device instances

[Michael S. Tsirkin]

On Sat, Jul 13, 2024 at 09:45:07PM +0900, Akihiko Odaki wrote:
> On 2024/07/10 19:52, Michael S. Tsirkin wrote:
> > On Wed, Jul 10, 2024 at 08:37:27AM +0200, Cédric Le Goater wrote:
> > > Hello,
> > > 
> > > This change introduced a regression on s390x. I could have spotted it
> > > earlier. Sorry about that. Here is the scenario,
> > > 
> > > QEMU now creates automatically the PCI device objects representing the
> > > VFs when the PF device is realized in pcie_sriov_pf_init(). This is
> > > good to report errors early but it has an important drawback.
> > > 
> > > On s390x, PCI devices have a dual S390PCIBusDevice object. This device
> > > model has 'uid' and 'fid' properties which can be either set by the VMM
> > > or, if not, auto-generated by the S390PCIBusDevice realize handler. In
> > > the VF case, these ids are auto-generated by QEMU and they can possibly
> > > conflict with the uid number space of libvirt. The conflict is detected
> > > when the machine is created and the start is aborted with a message :
> > > 
> > >    2024-07-08T12:51:42.876883Z qemu-system-s390x: -device 
> > > {"driver":"zpci","uid":17,"fid":16,"target":"hostdev0","id":"zpci17"}: 
> > > uid 17 already in use
> > > 
> > > This problem can occur today with a s390x VM using an IGB device.
> > > 
> > > It worked fine when the VFs were created at OS runtime because the initial
> > > topology of the machine was in place. Adding VFs was more or less like
> > > hotplug. AIUI, libvirt should have full control on the machine topology
> > > and so, creating VFs in QEMU at init time in the back of libvirt seems
> > > like a violation of this rule.
> > > 
> > > That said, the s390x case is specific and could perhaps be handled in a
> > > special way.
> > > 
> > > Thanks,
> > > 
> > > C.
> > 
> > 
> > Thanks for reporting this Cédric. Akihiko what's your
> > plan to handle this? Do you have the time to address this issue?
> 
> Creating VFs at initialization time only makes problems apparent early. Even
> without this change, hot-plugging another PCI device after realizing a VF
> results in a similar situation.
> 
> A proper way to handle this is to add new properties to igb and nvme to let
> libvirt specify the VF ids. However I wonder if it is a worthwhile addition
> (i.e., if igb and nvme's SR-IOV emulation will be used with s390x and
> libvirt).
> 
> Regards,
> Akihiko Odaki


Well okay but libvirt will not update overnight.
If we need time to discuss the design, I can revert for the
release and reapply after we have a fix.

-- 
MST