RE: [PATCH v1 15/22] Add iommufd configure option

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, September 21, 2023 2:20 AM
> 
> On Wed, Sep 20, 2023 at 12:17:24PM -0600, Alex Williamson wrote:
> 
> > > The iommufd design requires one open of the /dev/iommu to be shared
> > > across all the vfios.
> >
> > "requires"?  It's certainly of limited value to have multiple iommufd
> > instances rather than create multiple address spaces within a single
> > iommufd, but what exactly precludes an iommufd per device if QEMU, or
> > any other userspace so desired?  Thanks,
> 
> From the kernel side requires is too strong I suppose
> 

Agree. But with limited value let's stay with one iommufd per qemu
instance to reduce the maintenance burden.

It is also more future-friendly towards nested translation or the
need of centralized PASID tracking when supporting SIOV/ENQCMD, etc.

Supporting those new features between multiple iommufd's would
incur unnecessary complexities.