Re: [PATCH 2/3] iommu/arm-smmu: Add initial driver support for ARM SMMUv3 devices

[Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>]

Hi Will,

On Mon, Jun 01, 2015 at 10:40:14AM +0100, Will Deacon wrote:
> I like this proposal. The only remaining part is that the pgsize bitmap
> inherited by the domain can only truly be finalised once the page table
> is allocated, which in turn can only happen once we've identified the
> IOMMU instance.

We know the IOMMU instance for domains allocted with
iommu_domain_alloc_for_group because every group is behind a single
IOMMU instance. So the page-table can be allocated at domain creation
time and mappings can be created before the group itself is attached.

> In the case of iommu_domain_alloc_for_group, that's nice and
> straightforward (i.e. as part of the call) but for the default
> iommu_domain_alloc() case, we'd have to continue postponing things to
> ->attach time before we could provide a reliable pgsize_bitmap. This is
> to handle the case where an SMMU may be able to support only a subset of
> the pgsize_bitmap on a per-domain basis.

I don't think we need to postpone anything. Domains returned by
iommu_domain_alloc() need to work for all groups. If there are multiple
IOMMUs in the system with different capabilities/page-table formats the
IOMMU core needs to build multiple page-tables for that domain.

VFIO already does that as a workaround of how things are done currently,
but I really think this should be done in the IOMMU core code.

> In which situation do you think the merged pgsize_bitmap would get used?
> The only place I can think of is if we're trying to call iommu_map on a
> domain with no devices attached. However, if that domain was created
> using iommu_domain_alloc() then the pgsize_bitmap is the least of our
> worries -- we don't even know the page table format!

The first usecase for the merged pgsize_bitmap that comes to mind is to
determine the minimum page-size that can be used for a domain. In the
SMMUv3 case when there are IOMMUs with different minium page-sizes in one
system, this would be the biggest minimum page-size of all IOMMUs.

Probably there are other uses for that bitmap that show up at
implementation time.


	Joerg