[PATCH kernel] powerpc/iommu: Add iommu_ops to report capabilities and allow blocking domains

[Tian, Kevin]

> From: Alexey Kardashevskiy <aik at ozlabs.ru>
> Sent: Friday, July 29, 2022 11:50 AM
> 
> 
> On 29/07/2022 13:10, Tian, Kevin wrote:
> >> From: Oliver O'Halloran <oohall at gmail.com>
> >> Sent: Friday, July 29, 2022 10:53 AM
> >>
> >> On Fri, Jul 29, 2022 at 12:21 PM Alexey Kardashevskiy <aik at ozlabs.ru>
> wrote:
> >>>
> >>> *snip*
> >>>
> >>> About this. If a platform has a concept of explicit DMA windows (2 or
> >>> more), is it one domain with 2 windows or 2 domains with one window
> >> each?
> >>>
> >>> If it is 2 windows, iommu_domain_ops misses windows manipulation
> >>> callbacks (I vaguely remember it being there for embedded PPC64 but
> >>> cannot find it quickly).
> >>>
> >>> If it is 1 window per a domain, then can a device be attached to 2
> >>> domains at least in theory (I suspect not)?
> >>>
> >>> On server POWER CPUs, each DMA window is backed by an independent
> >> IOMMU
> >>> page table. (reminder) A window is a bus address range where devices
> are
> >>> allowed to DMA to/from ;)
> >>
> >> I've always thought of windows as being entries to a top-level "iommu
> >> page table" for the device / domain. The fact each window is backed by
> >> a separate IOMMU page table shouldn't really be relevant outside the
> >> arch/platform.
> >
> > Yes. This is what was agreed when discussing how to integrate iommufd
> > with POWER [1].
> >
> > One domain represents one address space.
> >
> > Windows are just constraints on the address space for what ranges can
> > be mapped.
> >
> > having two page tables underlying is just kind of POWER specific format.
> 
> 
> It is a POWER specific thing with one not-so-obvious consequence of each
> window having an independent page size (fixed at the moment or creation)
> and (most likely) different page size, like, 4K vs. 2M.
> 
> 

page size is anyway decided by the iommu driver. Same for other vendors.
the caller (e.g. vfio) just tries to map as many contiguous pages as
possible but in the end it's iommu driver to decide the actual size.