Re: [PATCH v9 7/8] xen/arm: enable dom0 to use PCI devices with pci-passthrough=no

[Julien Grall <julien@xxxxxxx>]

Hi,

On 28/04/2025 13:31, Mykyta Poturai wrote:
> On 28.04.25 11:54, Julien Grall wrote:
> > Hi Mykyta,
> >
> > On 14/03/2025 13:34, Mykyta Poturai wrote:
> > > From: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
> > >
> > > Enable the use of IOMMU + PCI in dom0 without having to specify
> > > "pci-passthrough=yes". We rely on dom0 to initialize the PCI controller
> > > and perform a PHYSDEVOP_pci_device_add call to add each device to SMMU.
> > It would be good to explain why Xen cannot initialize the PCI
> > controller. Asking, because the reason is the PCI controller is too
> > complex, then you will likely need the same approach for PCI passthrough...
> I think the main reason for this is complexity and the possibility of
> additional dependencies: there could be external clocks or reset pins
> that the PCI host depends on for working correctly. I will add this to
> the commit message. Regarding PCI passthrough, it is already using the
> same approach (at least on Arm). There are patches for enabling Xen on
> Arm to perform bus enumeration by itself by Luca Fancellu, but I haven't
> yet got to test them in a meaningful way.
Can you provide a link to the series? I would like to make sure we have 
a coherent approach. In particular, it is not clear to me how Dom0 and 
Xen will be able to coordinate the access to the PCI controller. Are we 
going to have a mediator?
> > > Enable pci_init() for initializing Xen's internal PCI subsystem, and
> > > allow PHYSDEVOP_pci_device_add when pci-passthrough is disabled.
> > Effectively, wouldn't this mean dom0 always *have* to call
> > PHYSDEVOP_pci_device_add? Otherwise, how would dom0 know whether it
> > needs to call PHSYDEVOP_pci_device_add?
> >
> > Cheers,
> Yes, I can't say for every system but with PCI host behind SMMU the
> PHYSDEVOP_pci_device_add call is required to use DMA.
Dom0 will not be able to know whether a device is protected by an IOMMU. 
So I guess it means the OS will need to be able to cope with an error 
(like on x86).
Cheers,

--
Julien Grall