From: Zach Pfeffer on
This patch contains the documentation for the API, termed the Virtual
Contiguous Memory Manager. Its use would allow all of the IOMMU to VM,
VM to device and device to IOMMU interoperation code to be refactored
into platform independent code.

Comments, suggestions and criticisms are welcome and wanted.

Signed-off-by: Zach Pfeffer <zpfeffer(a)>
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+What is this document about?
+This document covers how to use the Virtual Contiguous Memory Manager
+(VCMM), how the first implementation works with a specific low-level
+Input/Output Memory Management Unit (IOMMU) and the way the VCMM is used
+from user-space. It also contains a section that describes why something
+like the VCMM is needed in the kernel.
+If anything in this document is wrong, please send patches to the
+maintainer of this file, listed at the bottom of the document.
+The Virtual Contiguous Memory Manager
+The VCMM was built to solve the system-wide memory mapping issues that
+occur when many bus-masters have IOMMUs.
+An IOMMU maps device addresses to physical addresses. It also insulates
+the system from spurious or malicious device bus transactions and allows
+fine-grained mapping attribute control. The Linux kernel core does not
+contain a generic API to handle IOMMU mapped memory; device driver writers
+must implement device specific code to interoperate with the Linux kernel
+core. As the number of IOMMUs increases, coordinating the many address
+spaces mapped by all discrete IOMMUs becomes difficult without in-kernel
+The VCMM API enables device independent IOMMU control, virtual memory
+manager (VMM) interoperation and non-IOMMU enabled device interoperation
+by treating devices with or without IOMMUs and all CPUs with or without
+MMUs, their mapping contexts and their mappings using common
+abstractions. Physical hardware is given a generic device type and mapping
+contexts are abstracted into Virtual Contiguous Memory (VCM)
+regions. Users "reserve" memory from VCMs and "back" their reservations
+with physical memory.
+Why the VCMM is Needed
+Driver writers who control devices with IOMMUs must contend with device
+control and memory management. Driver writers have a large device driver
+API that they can leverage to control their devices, but they are lacking
+a unified API to help them program mappings into IOMMUs and share those
+mappings with other devices and CPUs in the system.
+Sharing is complicated by Linux's CPU-centric VMM. The CPU-centric model
+generally makes sense because average hardware only contains a MMU for the
+CPU and possibly a graphics MMU. If every device in the system has one or
+more MMUs the CPU-centric memory management (MM) programming model breaks
+Abstracting IOMMU device programming into a common API has already begun
+in the Linux kernel. It was built to abstract the difference between AMD
+and Intel IOMMUs to support x86 virtualization on both platforms. The
+interface is listed in include/linux/iommu.h. It contains
+interfaces for mapping and unmapping as well as domain management. This
+interface has not gained widespread use outside the x86; PA-RISC, Alpha
+and SPARC architectures and ARM and PowerPC platforms all use their own
+mapping modules to control their IOMMUs. The VCMM contains an IOMMU
+programming layer, but since its abstraction supports map management
+independent of device control, the layer is not used directly. This
+higher-level view enables a new kernel service, not just an IOMMU
+interoperation layer.
+The General Idea: Map Management using Graphs
+Looking at mapping from a system-wide perspective reveals a general graph
+problem. The VCMM's API is built to manage the general mapping graph. Each
+node that talks to memory, either through an MMU or directly (physically
+mapped) can be thought of as the device-end of a mapping edge. The other
+edge is the physical memory (or intermediate virtual space) that is
+In the direct-mapped case the device is assigned a one-to-one MMU. This
+scheme allows direct mapped devices to participate in general graph
+The CPU nodes can also be brought under the same mapping abstraction with
+the use of a light overlay on the existing VMM. This light overlay allows
+VMM-managed mappings to interoperate with the common API. The light
+overlay enables this without substantial modifications to the existing
+In addition to CPU nodes that are running Linux (and the VMM), remote CPU
+nodes that may be running other operating systems can be brought into the
+general abstraction. Routing all memory management requests from a remote
+node through the central memory management framework enables new features
+like system-wide memory migration. This feature may only be feasible for
+large buffers that are managed outside of the fast-path, but having remote
+allocation in a system enables features that are impossible to build
+without it.
+The fundamental objects that support graph-based map management are:
+1) Virtual Contiguous Memory Regions
+2) Reservations
+3) Associated Virtual Contiguous Memory Regions
+4) Memory Targets
+5) Physical Memory Allocations
+Usage Overview
+In a nutshell, users allocate Virtual Contiguous Memory Regions and
+associate those regions with one or more devices by creating an Associated
+Virtual Contiguous Memory Region. Users then create Reservations from the
+Virtual Contiguous Memory Region. At this point no physical memory has
+been committed to the reservation. To associate physical memory with a
+reservation a Physical Memory Allocation is created and the Reservation is
+backed with this allocation.
+include/linux/vcm.h includes comments documenting each API.
+Virtual Contiguous Memory Regions
+A Virtual Contiguous Memory Region (VCM) abstracts the memory space a
+device sees. The addresses of the region are only used by the devices
+which are associated with the region. This address space would normally be
+implemented as a device page table.
+A VCM is created and destroyed with three functions:
+ struct vcm *vcm_create(unsigned long start_addr, unsigned long len);
+ struct vcm *vcm_create_from_prebuilt(size_t ext_vcm_id);
+ int vcm_free(struct vcm *vcm);
+start_addr is an offset into the address space where allocations will
+start from. len is the length from start_addr of the VCM. Both functions
+generate an instance of a VCM.
+ext_vcm_id is used to pass a request to the VMM to generate a VCM
+instance. In the current implementation the call simply makes a note that
+the VCM instance is a VMM VCM instance for other interfaces usage. This
+muxing is seen throughout the implementation.
+vcm_create() and vcm_create_from_prebuilt() produce VCM instances for
+virtually mapped devices (IOMMUs and CPUs). To create a one-to-one mapped
+VCM, users pass the start_addr and len of the physical region. The VCMM
+matches this and records that the VCM instance is a one-to-one VCM.
+The newly created VCM instance can be passed to any function that needs to
+operate on or with a virtual contiguous memory region. Its main attributes
+are a start_addr and a len as well as an internal setting that allows the
+implementation to mux between true virtual spaces, one-to-one mapped
+spaces and VMM managed spaces.
+The current implementation uses the genalloc library to manage the VCM for
+IOMMU devices. Return values and more in-depth per-function documentation
+for these and the ones listed below are in include/linux/vcm.h.
+A Reservation is a contiguous region allocated from a VCM. There is no
+physical memory associated with it.
+A Reservation is created and destroyed with:
+ struct res *vcm_reserve(struct vcm *vcm, size_t len, u32 attr);
+ int vcm_unreserve(struct res *res);
+A vcm is a VCM created above. len is the length of the request. It can be
+up to the length of the VCM region the reservation is being created
+from. attr are mapping attributes: read, write, execute, user, supervisor,
+secure, not-cached, write-back/write-allocate, write-back/no
+write-allocate, write-through. These attrs are appropriate for ARM but can
+be changed to match to any architecture.
+The implementation calls gen_pool_alloc() for IOMMU devices,
+alloc_vm_area() for VMM areas and is a pass-through for one-to-one mapped
+Associated Virtual Contiguous Memory Regions and Activation
+An Associated Virtual Contiguous Memory Region (AVCM) is a mapping of a
+VCM to a device. The mapping can be active or inactive.
+An AVCM is managed with:
+ struct avcm *vcm_assoc(struct vcm *vcm, struct device *dev, u32 attr);
+ int vcm_deassoc(struct avcm *avcm);
+ int vcm_activate(struct avcm *avcm);
+ int vcm_deactivate(struct avcm *avcm);
+A VCM instance is a VCM created above. A dev is an opaque device handle
+thats passed down to the device driver the VCMM muxes in to handle a
+request. attr are association attributes: split, use-high or
+use-low. split controls which transactions hit a high-address page-table
+and which transactions hit a low-address page-table. For instance, all
+transactions whose most significant address bit is one would use the
+high-address page-table, any other transaction would use the low address
+page-table. This scheme is ARM-specific and could be changed in other
+architectures. One VCM instance can be associated with many devices and
+many VCM instances can be associated with one device.
+An AVCM is only a link. To program and deprogram a device with a VCM the
+user calls vcm_activate() and vcm_deactivate(). For IOMMU devices,
+activating a mapping programs the base address of a page table into an
+IOMMU. For VMM and one-to-one based devices, mappings are active
+immediately but the API does require an activation call for them for
+internal reference counting.
+Memory Targets
+A Memory Target is a platform independent way of specifying a physical
+pool; it abstracts a pool of physical memory. The physical memory pool may
+be physically discontiguous, need to be allocated from in a unique way or
+have other user-defined attributes.
+Physical Memory Allocation and Reservation Backing
+Physical memory is allocated as a separate step from reserving
+memory. This allows multiple reservations to back the same physical
+A Physical Memory Allocation is managed using the following functions:
+ struct physmem *vcm_phys_alloc(enum memtype_t memtype,
+ size_t len, u32 attr);
+ int vcm_phys_free(struct physmem *physmem);
+ int vcm_back(struct res *res, struct physmem *physmem);
+ int vcm_unback(struct res *res);
+attr can include an alignment request, a specification to map memory using
+various block sizes and/or to use physically contiguous memory. memtype is
+one of the memory types listed in Memory Targets.
+The current implementation manages two pools of memory. One pool is a
+contiguous block of memory and the other is a set of contiguous block
+pools. In the current implementation the block pools contain 4K, 64K and
+1M blocks. The physical allocator does not try to split blocks from the
+contiguous block pools to satisfy requests.
+The use of 4K, 64K and 1M blocks solves a problem with some IOMMU
+hardware. IOMMUs are placed in front of multimedia engines to provide a
+contiguous address space to the device. Multimedia devices need large
+buffers and large buffers may map to a large number of physical
+blocks. IOMMUs tend to have small translation lookaside buffers
+(TLBs). Since the TLB is small the number of physical blocks that map a
+given range needs to be small or else the IOMMU will continually fetch new
+translations during a typical streamed multimedia flow. By using a 1 MB
+mapping (or 64K mapping) instead of a 4K mapping the number of misses can
+be minimized, allowing the multimedia block to meet its performance goals.
+Low Level Control
+It is necessary in some instances to access attributes and provide
+higher-level control of the low-level hardware abstraction. The API
+contains many members and functions for this task but the two that are
+typically used are:
+ res->dev_addr;
+ int vcm_hook(struct device *dev, vcm_handler handler, void *data);
+res->dev_addr is the device address given a reservation. This device
+address is a virtual IOMMU address for reservations on IOMMU VCMs, a
+virtual VMM address for reservations on VMM VCMs and a virtual (really
+physical since its one-to-one mapped) address for one-to-one devices.
+The function, vcm_hook, allows a caller in the kernel to register a
+user_handler. The handler is passed the data member passed to vcm_hook
+during a fault. The user can return 1 to indicate that the underlying
+driver should handle the fault and retry the transaction or they can
+return 0 to halt the transaction. If the user doesn't register a
+handler the low-level driver will print a warning and terminate the
+A Detailed Walk Through
+The following call sequence walks through a typical allocation
+sequence. In the first stage the memory for a device is reserved and
+backed. This occurs without mapping the memory into a VMM VCM region. The
+second stage maps the first VCM region into a VMM VCM region so the kernel
+can read or write it. The second stage is not necessary if the VMM does
+not need to read or modify the contents of the original mapping.
+ Stage 1: Map and Allocate Memory for a Device
+ The call sequence starts by creating a VCM region:
+ vcm = vcm_create(start_addr, len);
+ The next call associates a VCM region with a device:
+ avcm = vcm_assoc(vcm, dev, attr);
+ To activate the association, users call vcm_activate() on the avcm from
+ the associate call. This programs the underlining device with the
+ mappings.
+ ret = vcm_activate(avcm);
+ Once a VCM region is created and associated it can be reserved from
+ with:
+ res = vcm_reserve(vcm, res_len, res_attr);
+ A user then allocates physical memory with:
+ physmem = vcm_phys_alloc(memtype, len, phys_attr);
+ To back the reservation with the physical memory allocation the user
+ calls:
+ vcm_back(res, physmem);
+ Stage 2: Map the Device's Memory into the VMM's VCM region
+ If the VMM needs to read and/or write the region that was just created,
+ the following calls are made.
+ The first call creates a prebuilt VCM with:
+ vcm_vmm = vcm_from_prebuilt(ext_vcm_id);
+ The prebuilt VCM is associated with the CPU device and activated with:
+ avcm_vmm = vcm_assoc(vcm_vmm, dev_cpu, attr);
+ vcm_activate(avcm_vmm);
+ A reservation is made on the VMM VCM with:
+ res_vmm = vcm_reserve(vcm_vmm, res_len, attr);
+ Finally, once the topology has been set up a vcm_back() allows the VMM
+ to read the memory using the physmem generated in stage 1:
+ vcm_back(res_vmm, physmem);
+Mapping IOMMU, one-to-one and VMM Reservations
+The following example demonstrates mapping IOMMU, one-to-one and VMM
+reservations to the same physical memory. It shows the use of phys_addr
+and phys_size to create a contiguous VCM for one-to-one mapped devices.
+ The user allocates physical memory:
+ physmem = vcm_phys_alloc(memtype, SZ_2MB + SZ_4K, CONTIGUOUS);
+ Creates an IOMMU VCM:
+ vcm_iommu = vcm_create(SZ_1K, SZ_16M);
+ Creates a one-to-one VCM:
+ vcm_onetoone = vcm_create(phys_addr, phys_size);
+ Creates a Prebuit VCM:
+ vcm_vmm = vcm_from_prebuit(ext_vcm_id);
+ Associate and activate all three to their respective devices:
+ avcm_iommu = vcm_assoc(vcm_iommu, dev_iommu, attr0);
+ avcm_onetoone = vcm_assoc(vcm_onetoone, dev_onetoone, attr1);
+ avcm_vmm = vcm_assoc(vcm_vmm, dev_cpu, attr2);
+ vcm_activate(avcm_iommu);
+ vcm_activate(avcm_onetoone);
+ vcm_activate(avcm_vmm);
+ Associations that fail return 0.
+ And finally, creates and backs reservations on all 3 such that they
+ all point to the same memory:
+ res_iommu = vcm_reserve(vcm_iommu, SZ_2MB + SZ_4K, attr);
+ res_onetoone = vcm_reserve(vcm_onetoone, SZ_2MB + SZ_4K, attr);
+ res_vmm = vcm_reserve(vcm_vmm, SZ_2MB + SZ_4K, attr);
+ vcm_back(res_iommu, physmem);
+ vcm_back(res_onetoone, physmem);
+ vcm_back(res_vmm, physmem);
+ Like associations, reservations that fail return 0.
+VCM Summary
+The VCMM is an attempt to abstract attributes of three distinct classes of
+mappings into one API. The VCMM allows users to reason about mappings as
+first class objects. It also allows memory mappings to flow from the
+traditional 4K mappings prevalent on systems today to more efficient block
+sizes. Finally, it allows users to manage mapping interoperation without
+becoming VMM experts. These features will allow future systems with many
+MMU mapped devices to interoperate simply and therefore correctly.
+IOMMU Hardware Control
+The VCM currently supports a single type of IOMMU, a Qualcomm System MMU
+(SMMU). The SMMU interface contains functions to map and unmap virtual
+addresses, perform address translations and initialize hardware. A
+Qualcomm SMMU can contain multiple MMU contexts. Each context can
+translate in parallel. All contexts in a SMMU share one global translation
+look-aside buffer (TLB).
+To support context muxing the SMMU module creates and manages device
+independent virtual contexts. These context abstractions are bound to
+actual contexts at run-time. Once bound, a context can be activated. This
+activation programs the underlying context with the virtual context
+affecting a context switch.
+The following functions are all documented in:
+ arch/arm/mach-msm/include/mach/smmu_driver.h.
+To map and unmap a virtual page into physical space the VCM calls:
+ int smmu_map(struct smmu_dev *dev, unsigned long pa,
+ unsigned long va, unsigned long len, unsigned int attr);
+ int smmu_unmap(struct smmu_dev *dev, unsigned long va,
+ unsigned long len);
+ int smmu_update_start(struct smmu_dev *dev);
+ int smmu_update_done(struct smmu_dev *dev);
+The size given to map must be 4K, 64K, 1M or 16M and the VA and PA must be
+aligned to the given size. smmu_update_start() and smmu_update_done()
+should be called before and after each map or unmap.
+To request a hardware VA to PA translation on a single address the VCM
+ unsigned long smmu_translate(struct smmu_dev *dev,
+ unsigned long va);
+Fault Handling
+To register an interrupt handler for a context the VCM calls:
+ int smmu_hook_interrupt(struct smmu_dev *dev, vcm_handler handler,
+ void *data);
+The registered interrupt handler should return 1 if it wants the SMMU
+driver to retry the transaction again and 0 if it wants the SMMU driver to
+terminate the transaction.
+Managing SMMU Initialization and Contexts
+SMMU hardware initialization and management happens in 2 steps. The first
+step initializes global SMMU devices and abstract device contexts. The
+second step binds contexts and devices.
+An SMMU hardware instance is built with:
+ int smmu_drvdata_init(struct smmu_driver *drv, unsigned long base,
+ int irq);
+An SMMU context is initialized and deinitialized with:
+ struct smmu_dev *smmu_ctx_init(int ctx);
+ int smmu_ctx_deinit(struct smmu_dev *dev);
+An abstract SMMU context is bound to a particular SMMU with:
+ int smmu_ctx_bind(struct smmu_dev *ctx, struct smmu_driver *drv);
+Activation affects a context switch.
+Activation, deactivation and activation state testing are done with:
+ int smmu_activate(struct smmu_dev *dev);
+ int smmu_deactivate(struct smmu_dev *dev);
+ int smmu_is_active(struct smmu_dev *dev);
+Userspace Access to Devices with IOMMUs
+A device that issues transactions through an IOMMU must work with two
+APIs. The first API is the VCM. The VCM API is device independent. Users
+pass the VCM a dev_id and the VCM makes calls on the hardware device it
+has been configured with using this dev_id. The second API is whatever
+device topology has been created to organize the particular IOMMUs in a
+system. The only constraint on this second API is that it must give the
+user a single dev_id that it can pass through the VCM.
+For the Qualcomm SMMUs the second API consists of a tree of platform
+devices and two platform drivers as well as a context lookup function that
+traverses the device tree and returns a dev_id given a context name.
+Qualcomm SMMU Device Tree
+The current tree organizes the devices into a tree that looks like the
+ smmu0/
+ ctx0
+ ctx1
+ ctx2
+ smmu1/
+ ctx3
+Each context, ctx[n] and each smmu, smmu[n] is given a name. Since users
+are interested in contexts not smmus, the context name is passed to a
+function to find the dev_id associated with that name. The functions to
+find, free and get the base address (since the device probe function calls
+ioremap to map the SMMUs configuration registers into the kernel) are
+listed here:
+ struct smmu_dev *smmu_get_ctx_instance(char *ctx_name);
+ int smmu_free_ctx_instance(struct smmu_dev *dev);
+ unsigned long smmu_get_base_addr(struct smmu_dev *dev);
+Documentation for these functions is in:
+ arch/arm/mach-msm/include/mach/smmu_device.h
+Each context is given a dev node named after the context. For example:
+ /dev/vcodec_a_mm1
+ /dev/vcodec_b_mm2
+ /dev/vcodec_stream
+ etc...
+Users open, close and mmap these nodes to access VCM buffers from
+userspace in the same way that they used to open, close and mmap /dev
+nodes that represented large physically contiguous buffers (called PMEM
+buffers on Android).
+An abbreviated example is shown here:
+Users get the dev_id associated with their target context, create a VCM
+topology appropriate for their device and finally associate the VCMs of
+the topology with the contexts that will take the VCMs:
+ dev_id = smmu_get_ctx_instance(vcodec_a_stream);
+create vcm and needed topology
+ avcm = vcm_assoc(vcm, dev_id, attr);
+Tying it all Together
+VCMs, IOMMUs and the device tree all work to support system-wide memory
+mappings. The use of each API in this system allows users to concentrate
+on the relevant details without needing to worry about low-level
+details. The API's clear separation of memory spaces and the devices that
+support those memory spaces continues the Linux tradition of abstracting the
+what from the how.
+Maintainer: Zach Pfeffer <zpfeffer(a)>

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