Re: [GIT PULL] iommu: Kill off pgsize_bitmap field from struct iommu_ops

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

Hi Will,

On Tue, Mar 31, 2015 at 03:49:56PM +0100, Will Deacon wrote:
> But isn't this restriction already the case in practice? For example, if
> I have a domain with some mapping already configured, then that mapping
> will be using some fixed set of page sizes. Attaching a device behind
> another IOMMU that doesn't support that page size would effectively require
> the domain page tables to be freed and re-allocated from scratch.

The problem is that this restriction depends on the IOMMU driver in use.
>From the beginning of the IOMMU-API the backend drivers have supported
sharing a domain by multiple devices. In fact, the domain is just an
abstraction for an address space that can be attached to devices just
like cpu page-tables are assigned to threads of a process.

We can discuss whether this fundamental concept of the IOMMU-API needs
to be changed (moving into the direction this patch-set proposes). I
could imaging removing the domain concept entirely and just have
map/unmap functions like this:

	iommu_map(struct device *dev, dma_addr_t iova,
		  phys_addr_t phys, size_t size)
	iommu_unmap(struct device *dev, dma_addr_t iova,
		    phys_addr_t phys, size_t size)

This would require some changes for the users of the IOMMU-API, but it
shouldn't be too hard.

Or we keep the desired semantics as they are now and do everything
needed for that in the IOMMU drivers. For the arm-smmu this would mean
exposing a common set of supported pgsizes between IOMMUs, or to build
multiple page-tables to match the different IOMMU capabilities.

I am open for disussions for either way, but I like the current
semantics a bit more, as it allows us to share page-tables between
devices and we can move all of the nasty code in VFIO that already
creates multiple domains to get different page-tables into the
IOMMU core or the drivers (were it belongs).

What I definitly don't want is a mixture of both concepts depending on
the IOMMU driver in use. We should settle on one way and force the
drivers to behave accordingly. We don't need a common API when every
driver behaves differently.


	Joerg