Re: [PATCH 0/2] s390x/pci: relax I/O address translation requirement

[Thomas Huth]

On 12/12/2024 15.42, Matthew Rosato wrote:
> On 12/12/24 4:10 AM, Thomas Huth wrote:
> > On 09/12/2024 20.29, Matthew Rosato wrote:
> > > This series introduces the concept of the relaxed translation requirement
> > > for s390x guests in order to allow bypass of the guest IOMMU for more
> > > efficient PCI passthrough.
> > >
> > > With this series, QEMU can indicate to the guest that an IOMMU is not
> > > strictly required for a zPCI device.  This would subsequently allow a
> > > guest linux to use iommu.passthrough=1 and bypass their guest IOMMU for
> > > PCI devices.
> > >
> > > When this occurs, QEMU will note the behavior via an intercepted MPCIFC
> > > instruction and will fill the host iommu with mappings of the entire
> > > guest address space in response.
> > >
> > > There is a kernel series [1] that adds the relevant behavior needed to
> > > exploit this new feature from within a s390x linux guest.
> > >
> > > [1]: 
> > > https://lore.kernel.org/linux-s390/20241209192403.107090-1-mjrosato@linux.ibm.com/
> > >
> > > Matthew Rosato (2):
> > >     s390x/pci: add support for guests that request direct mapping
> > >     s390x/pci: indicate QEMU supports relaxed translation for passthrough
> >
> >   Hi again!
> >
> > One more thought: This is a guest-visible feature, isn't it? So do we also need 
> > some migration handling for this? For example, what happens if you start a 
> > guest that is aware of this feature on a host that has a QEMU with this 
> > feature, and then try to live-migrate the guest to a QEMU that does not have 
> > this feature? I guess the guest will crash? It would be better to fail the 
> > migration instead. At least we should disable the feature in older machine 
> > types and only allow it for the latest one.
>
> zPCI devices are currently marked as unmigratable in s390_pci_device_vmstate so 
> it's not a reproducible issue yet.

Ah, right, I forgot about that migration blocker, so we should be fine, indeed!

> Re: disabling the feature for older machines, OK -- Shall I fence similar to what we did 
> for interpret/forwarding-assist with a new device property that is default to off on 
> older machines ("relax-translation"? alternative suggestions welcome)

Sounds reasonable!

 Thomas