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

On 29/07/2022 13:10, Tian, Kevin wrote:

quoted

From: Oliver O'Halloran <oohall@gmail.com>
Sent: Friday, July 29, 2022 10:53 AM

On Fri, Jul 29, 2022 at 12:21 PM Alexey Kardashevskiy [off-list ref] wrote:

quoted

*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?

quoted

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

quoted

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.

Thanks
Kevin

[1] https://lore.kernel.org/all/Yns+TCSa6hWbU7wZ@yekko/ (local)

-- 
Alexey