[RFC] Generic dma_ops using iommu-api - some thoughts

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Hi,

as promised here is a write-up of my thoughts about implementing generic
dma_ops on-top of the IOMMU-API and what is required for that. I am
pretty sure I forgot some people on the Cc-list, so if anybody is
missing feel free to add her/him.

All kinds of useful comments appreciated, too :-)

Okay, here is the text:

Some Thoughts About a Generic DMA-API Implemention Using IOMMU-API
=======================================================================

This document describes some ideas about a generic implementation for
the DMA-API which only uses the IOMMU-API as its backend. Many IOMMU
drivers for Linux exist and they all implement their own implementation
for the DMA-API. A generic implementation would allow to put all
hardware specifics into the IOMMU-API and factor out the common code.

Types of IOMMUs
-----------------------------------------------------------------------

Most IOMMUs around fit in one of two categories:

Type 1: I call these GART-like IOMMUs. These IOMMUs provide an aperture
        range which can be remapped by a page-table (often single-level)
	This type of IOMMU exists on different architectures and there
	are also multiple hardware variants of them on the same
	architecture.
	These IOMMUs have no or only limited support for
	device-isolation. The different hardware implementations vary in
	some side-parameters like the size of the aperture and whether
	devices are allowed to use addresses outside of the aperture.

Type 2: Full-isolation capable IOMMUs. There are only two of them known
        to me: VT-d and AMD-Vi. These IOMMUs support a full 64 bit
	device address space and have support for full-isolation. This
	means that they can configure a seperate address space for each
	device.
	These IOMMUs may also have support for Interrupt remapping. But
	this feature is not subject of the IOMMU-API.

Differences between DMA-API and IOMMU-API
-----------------------------------------------------------------------

The difference between these two APIs is basically the scope. The
IOMMU-API only cares about address remapping for devices. This proposal
does not intend to change that.
The scope of the DMA-API is to provide dma handles for device drivers
and to maintain the coherency between device and cpu view of memory. So
the scope of the DMA-API is much larger. From an implementation pov it
looks like that:

	IOMMU-API <-------------------- DMA-API
	(hardare access and		(implements address allocator
	 remapping setup)		 and maintains cache coherency)

The IOMMU-API
-----------------------------------------------------------------------

The API to support IOMMUs does only handle type 2. This was sufficient
when the IOMMU-API was introduced because the only reason was to provide
device-passthrough support for KVM.
When we want to write a a DMA-API layer on-top of that API is makes a
lot of sense to extend it to type 1 because most IOMMUs belong to that
type.
Lets first look what the IOMMU-API provides today. A domain is an
abstraction for a device address space. The most important
data-structure there-in is the page-table.

iommu_found()		All other functions can only called safely when
			this returns true
iommu_domain_alloc()    Allocates a new domain
iommu_domain_free()	Destroys a domain
iommu_attach_device()   Put a device into a given domain
iommu_detach_device()   Removes a device from a given domain
iommu_map()		Maps a given system physical address to a given
			io virtual address in one domain
iommu_unmap()		Removes a mapping from a domain
iommu_iova_to_phys()    Returns physical address for a io virtual one if
			it exists
iommu_domain_has_cap()	Check for IOMMU capablilities. Only used for
			PCIe snoop-bit forcing today

Changes to the IOMMU-API
-----------------------------------------------------------------------

The current assumption about a domain is that any io virtual address can
be mapped to any system physical address. This can not longer be assumed
when type 1 IOMMUs are supported. The part of the io address space that
can be remapped may be very small (ususally 64MB for an AMD NB-GART) and
may not start at address zero. Additional function(s) are needed so that
the DMA-API implementation can query these properties from a domain.

Further it is currently undefined in which domain a device is per
default. For supporting the DMA-API every device needs to be put into a
default domain by the IOMMU driver. This domain is then used by the
DMA-API code.

The DMA-API manages the address allocator, so it needs to keep track of
the allocator state for each domain. This can be solved by storing a
private pointer into a domain.

Also, the IOMMU driver may need to put multiple devices into the same
domain. This is necessary for type 2 IOMMUs too because the hardware
may not be able to distinguisch between all devices (so it is usually
not possible to distinguish between different 32-bit PCI devices on the
same bus). Support for different domains is even more limited on type 1
IOMMUs. The AMD NB-GART supports only one domain for all devices.
Therefore it may be helpful to find the domain associated with one
device. This is also needed for the DMA-API to get a pointer to the
default domain for each device.

With these changes I think we can handle type 1 and 2 IOMMUs in the
IOMMU-API and use it as a basis for the DMA-API. The IOMMU driver
provides a default domain which contains an aperture where addresses can
be remapped. Type 2 IOMMUs can provide apertures that cover the whole
address space or emulate a type 1 IOMMU by providing a smaller aperture.
The IOMMU driver also provides the capabilities of the aperture like if
it is possible to use addresses outside of the aperture directly.

DMA-API Considerations
-----------------------------------------------------------------------

The question here is which address allocator should be implemented.
Almost all IOMMU drivers today implement a bitmap based allocator. This
one has advantages because it is very simple, has proven existing code
which can be reused and allows neat optimizations in IOMMU TLB flushing.
Flushing the TLB of an IOMMU is usually an expensive operation.

On the other hand the bitmap allocator does not scale very well with the
size of the remapable area. Therefore the VT-d driver implements a
tree-based allocator which can handle a large address space efficiently,
but does not allow to optimize IO/TLB flushing.

It remains to be determined which allocator algortihm fits best.


Regards,

	Joerg


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