Re: [PATCH v2 06/19] iommufd/viommu: Add IOMMU_VIOMMU_SET/UNSET_VDEV_ID ioctl

[Mostafa Saleh <smostafa@google.com>]

On Fri, Sep 27, 2024 at 11:01:41AM -0300, Jason Gunthorpe wrote:
> On Fri, Sep 27, 2024 at 01:50:52PM +0000, Mostafa Saleh wrote:
> 
> > My understanding of IOMMUFD is very little, but AFAICT, that means that
> > it’s assumed that each device can only have one stream ID(RID)?
> > 
> > As I can see in patch 17 in arm_smmu_convert_viommu_vdev_id(), it
> > converts the virtual ID to a physical one using master->streams[0].id.
> > 
> > Is that correct or am I missing something?
> > 
> > As I am looking at similar problem for paravirtual IOMMU with pKVM, where
> > the UAPI would be something similar to:
> > 
> > 	GET_NUM_END_POINTS(dev) => nr_sids
> > 
> > 	SET_END_POINT_VSID(dev, sid_index, vsid)
> > 
> > Similar to what VFIO does with IRQs.
> > 
> > As a device can have many SIDs.
> 
> We don't support multi SID through this interface, at least in this
> version.
> 
> To do multi-sid you have to inform the VM of all the different pSIDs
> the device has and then setup the vSID/pSID translation to map them
> all to the HW invalidation logic.

Why would the VM need to know the pSID? The way I view this is quite close to
how irq works, the VM only views the GSI which is the virtualized number.
The VMM then would need to configure vSID->pSID translation, also without
knowing the actual pSID, just how many SIDs are there per-device; very similar to
how it configures IRQs through
VFIO_DEVICE_GET_INFO/VFIO_DEVICE_SET_IRQS.

And as long as we only allow 1:1 vSID to pSID mapping, I guess it would be
easy to implement.

> 
> Which is alot more steps, and we have no use case right now. Multi-sid
> is also not something I expect to see in any modern PCI device, and
> this is VFIO PCI...
> 

Ah, I thought IOMMUFD would be used instead of VFIO_TYPE1*, which should cover
platform devices (VFIO-platform) or am I missing something?

And multi-SIDs is common in platform devices and this would be quite
restricting, and I was hoping to support the pKVM vIOMMU through IOMMUFD
interface.

If possible, can the UAPI be designed with this in mind, even if not
implemented now?

Thanks,
Mostafa

> Jason