Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable nested SMMUv3

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Jan 31, 2025 at 02:39:53PM +0000, Shameerali Kolothum Thodi wrote:

> > > And Qemu does some checking to make sure that the device is indeed
> > associated
> > > with the specified phys-smmuv3.  This can be done going through the
> > sysfs path checking
> > > which is what I guess libvirt is currently doing to populate the topology.
> > So basically
> > > Qemu is just replicating that to validate again.
> > 
> > I would prefer that iommufd users not have to go out to sysfs..
> > 
> > > Or another option is extending the IOMMU_GET_HW_INFO IOCTL to
> > return the phys
> > > smmuv3 base address which can avoid going through the sysfs.
> > 
> > It also doesn't seem great to expose a physical address. But we could
> > have an 'iommu instance id' that was a unique small integer?
> 
> Ok. But how the user space can map that to the device?

Why does it need to?

libvirt picks some label for the vsmmu instance, it doesn't matter
what the string is.

qemu validates that all of the vsmmu instances are only linked to PCI
device that have the same iommu ID. This is already happening in the
kernel, it will fail attaches to mismatched instances.

Nothing further is needed?

Jason