IOMMUFD

Author

Jason Gunthorpe

Author

Kevin Tian

Overview

IOMMUFD is the user API to control the IOMMU subsystem as it relates to managing IO page tables from userspace using file descriptors. It intends to be general and consumable by any driver that wants to expose DMA to userspace. These drivers are eventually expected to deprecate any internal IOMMU logic they may already/historically implement (e.g. vfio_iommu_type1.c).

At minimum iommufd provides universal support of managing I/O address spaces and I/O page tables for all IOMMUs, with room in the design to add non-generic features to cater to specific hardware functionality.

In this context the capital letter (IOMMUFD) refers to the subsystem while the small letter (iommufd) refers to the file descriptors created via /dev/iommu for use by userspace.

Key Concepts

User Visible Objects

Following IOMMUFD objects are exposed to userspace:

  • IOMMUFD_OBJ_IOAS, representing an I/O address space (IOAS), allowing map/unmap of user space memory into ranges of I/O Virtual Address (IOVA).

    The IOAS is a functional replacement for the VFIO container, and like the VFIO container it copies an IOVA map to a list of iommu_domains held within it.

  • IOMMUFD_OBJ_DEVICE, representing a device that is bound to iommufd by an external driver.

  • IOMMUFD_OBJ_HW_PAGETABLE, representing an actual hardware I/O page table (i.e. a single struct iommu_domain) managed by the iommu driver.

    The IOAS has a list of HW_PAGETABLES that share the same IOVA mapping and it will synchronize its mapping with each member HW_PAGETABLE.

All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.

The diagram below shows relationship between user-visible objects and kernel datastructures (external to iommufd), with numbers referred to operations creating the objects and links:

 _________________________________________________________
|                         iommufd                         |
|       [1]                                               |
|  _________________                                      |
| |                 |                                     |
| |                 |                                     |
| |                 |                                     |
| |                 |                                     |
| |                 |                                     |
| |                 |                                     |
| |                 |        [3]                 [2]      |
| |                 |    ____________         __________  |
| |      IOAS       |<--|            |<------|          | |
| |                 |   |HW_PAGETABLE|       |  DEVICE  | |
| |                 |   |____________|       |__________| |
| |                 |         |                   |       |
| |                 |         |                   |       |
| |                 |         |                   |       |
| |                 |         |                   |       |
| |                 |         |                   |       |
| |_________________|         |                   |       |
|         |                   |                   |       |
|_________|___________________|___________________|_______|
          |                   |                   |
          |              _____v______      _______v_____
          | PFN storage |            |    |             |
          |------------>|iommu_domain|    |struct device|
                        |____________|    |_____________|
  1. IOMMUFD_OBJ_IOAS is created via the IOMMU_IOAS_ALLOC uAPI. An iommufd can hold multiple IOAS objects. IOAS is the most generic object and does not expose interfaces that are specific to single IOMMU drivers. All operations on the IOAS must operate equally on each of the iommu_domains inside of it.

  2. IOMMUFD_OBJ_DEVICE is created when an external driver calls the IOMMUFD kAPI to bind a device to an iommufd. The driver is expected to implement a set of ioctls to allow userspace to initiate the binding operation. Successful completion of this operation establishes the desired DMA ownership over the device. The driver must also set the driver_managed_dma flag and must not touch the device until this operation succeeds.

  3. IOMMUFD_OBJ_HW_PAGETABLE is created when an external driver calls the IOMMUFD kAPI to attach a bound device to an IOAS. Similarly the external driver uAPI allows userspace to initiate the attaching operation. If a compatible pagetable already exists then it is reused for the attachment. Otherwise a new pagetable object and iommu_domain is created. Successful completion of this operation sets up the linkages among IOAS, device and iommu_domain. Once this completes the device could do DMA.

    Every iommu_domain inside the IOAS is also represented to userspace as a HW_PAGETABLE object.

    Note

    Future IOMMUFD updates will provide an API to create and manipulate the HW_PAGETABLE directly.

A device can only bind to an iommufd due to DMA ownership claim and attach to at most one IOAS object (no support of PASID yet).

Kernel Datastructure

User visible objects are backed by following datastructures:

  • iommufd_ioas for IOMMUFD_OBJ_IOAS.

  • iommufd_device for IOMMUFD_OBJ_DEVICE.

  • iommufd_hw_pagetable for IOMMUFD_OBJ_HW_PAGETABLE.

Several terminologies when looking at these datastructures:

  • Automatic domain - refers to an iommu domain created automatically when attaching a device to an IOAS object. This is compatible to the semantics of VFIO type1.

  • Manual domain - refers to an iommu domain designated by the user as the target pagetable to be attached to by a device. Though currently there are no uAPIs to directly create such domain, the datastructure and algorithms are ready for handling that use case.

  • In-kernel user - refers to something like a VFIO mdev that is using the IOMMUFD access interface to access the IOAS. This starts by creating an iommufd_access object that is similar to the domain binding a physical device would do. The access object will then allow converting IOVA ranges into struct page * lists, or doing direct read/write to an IOVA.

iommufd_ioas serves as the metadata datastructure to manage how IOVA ranges are mapped to memory pages, composed of:

  • struct io_pagetable holding the IOVA map

  • struct iopt_area's representing populated portions of IOVA

  • struct iopt_pages representing the storage of PFNs

  • struct iommu_domain representing the IO page table in the IOMMU

  • struct iopt_pages_access representing in-kernel users of PFNs

  • struct xarray pinned_pfns holding a list of pages pinned by in-kernel users

Each iopt_pages represents a logical linear array of full PFNs. The PFNs are ultimately derived from userspace VAs via an mm_struct. Once they have been pinned the PFNs are stored in IOPTEs of an iommu_domain or inside the pinned_pfns xarray if they have been pinned through an iommufd_access.

PFN have to be copied between all combinations of storage locations, depending on what domains are present and what kinds of in-kernel "software access" users exist. The mechanism ensures that a page is pinned only once.

An io_pagetable is composed of iopt_areas pointing at iopt_pages, along with a list of iommu_domains that mirror the IOVA to PFN map.

Multiple io_pagetable-s, through their iopt_area-s, can share a single iopt_pages which avoids multi-pinning and double accounting of page consumption.

iommufd_ioas is shareable between subsystems, e.g. VFIO and VDPA, as long as devices managed by different subsystems are bound to a same iommufd.

IOMMUFD User API

General ioctl format

The ioctl interface follows a general format to allow for extensibility. Each ioctl is passed in a structure pointer as the argument providing the size of the structure in the first u32. The kernel checks that any structure space beyond what it understands is 0. This allows userspace to use the backward compatible portion while consistently using the newer, larger, structures.

ioctls use a standard meaning for common errnos:

  • ENOTTY: The IOCTL number itself is not supported at all

  • E2BIG: The IOCTL number is supported, but the provided structure has non-zero in a part the kernel does not understand.

  • EOPNOTSUPP: The IOCTL number is supported, and the structure is understood, however a known field has a value the kernel does not understand or support.

  • EINVAL: Everything about the IOCTL was understood, but a field is not correct.

  • ENOENT: An ID or IOVA provided does not exist.

  • ENOMEM: Out of memory.

  • EOVERFLOW: Mathematics overflowed.

As well as additional errnos, within specific ioctls.

struct iommu_destroy

ioctl(IOMMU_DESTROY)

Definition:

struct iommu_destroy {
    __u32 size;
    __u32 id;
};

Members

size

sizeof(struct iommu_destroy)

id

iommufd object ID to destroy. Can be any destroyable object type.

Description

Destroy any object held within iommufd.

struct iommu_ioas_alloc

ioctl(IOMMU_IOAS_ALLOC)

Definition:

struct iommu_ioas_alloc {
    __u32 size;
    __u32 flags;
    __u32 out_ioas_id;
};

Members

size

sizeof(struct iommu_ioas_alloc)

flags

Must be 0

out_ioas_id

Output IOAS ID for the allocated object

Description

Allocate an IO Address Space (IOAS) which holds an IO Virtual Address (IOVA) to memory mapping.

struct iommu_iova_range

ioctl(IOMMU_IOVA_RANGE)

Definition:

struct iommu_iova_range {
    __aligned_u64 start;
    __aligned_u64 last;
};

Members

start

First IOVA

last

Inclusive last IOVA

Description

An interval in IOVA space.

struct iommu_ioas_iova_ranges

ioctl(IOMMU_IOAS_IOVA_RANGES)

Definition:

struct iommu_ioas_iova_ranges {
    __u32 size;
    __u32 ioas_id;
    __u32 num_iovas;
    __u32 __reserved;
    __aligned_u64 allowed_iovas;
    __aligned_u64 out_iova_alignment;
};

Members

size

sizeof(struct iommu_ioas_iova_ranges)

ioas_id

IOAS ID to read ranges from

num_iovas

Input/Output total number of ranges in the IOAS

__reserved

Must be 0

allowed_iovas

Pointer to the output array of struct iommu_iova_range

out_iova_alignment

Minimum alignment required for mapping IOVA

Description

Query an IOAS for ranges of allowed IOVAs. Mapping IOVA outside these ranges is not allowed. num_iovas will be set to the total number of iovas and the allowed_iovas[] will be filled in as space permits.

The allowed ranges are dependent on the HW path the DMA operation takes, and can change during the lifetime of the IOAS. A fresh empty IOAS will have a full range, and each attached device will narrow the ranges based on that device's HW restrictions. Detaching a device can widen the ranges. Userspace should query ranges after every attach/detach to know what IOVAs are valid for mapping.

On input num_iovas is the length of the allowed_iovas array. On output it is the total number of iovas filled in. The ioctl will return -EMSGSIZE and set num_iovas to the required value if num_iovas is too small. In this case the caller should allocate a larger output array and re-issue the ioctl.

out_iova_alignment returns the minimum IOVA alignment that can be given to IOMMU_IOAS_MAP/COPY. IOVA's must satisfy:

starting_iova % out_iova_alignment == 0
(starting_iova + length) % out_iova_alignment == 0

out_iova_alignment can be 1 indicating any IOVA is allowed. It cannot be higher than the system PAGE_SIZE.

struct iommu_ioas_allow_iovas

ioctl(IOMMU_IOAS_ALLOW_IOVAS)

Definition:

struct iommu_ioas_allow_iovas {
    __u32 size;
    __u32 ioas_id;
    __u32 num_iovas;
    __u32 __reserved;
    __aligned_u64 allowed_iovas;
};

Members

size

sizeof(struct iommu_ioas_allow_iovas)

ioas_id

IOAS ID to allow IOVAs from

num_iovas

Input/Output total number of ranges in the IOAS

__reserved

Must be 0

allowed_iovas

Pointer to array of struct iommu_iova_range

Description

Ensure a range of IOVAs are always available for allocation. If this call succeeds then IOMMU_IOAS_IOVA_RANGES will never return a list of IOVA ranges that are narrower than the ranges provided here. This call will fail if IOMMU_IOAS_IOVA_RANGES is currently narrower than the given ranges.

When an IOAS is first created the IOVA_RANGES will be maximally sized, and as devices are attached the IOVA will narrow based on the device restrictions. When an allowed range is specified any narrowing will be refused, ie device attachment can fail if the device requires limiting within the allowed range.

Automatic IOVA allocation is also impacted by this call. MAP will only allocate within the allowed IOVAs if they are present.

This call replaces the entire allowed list with the given list.

enum iommufd_ioas_map_flags

Flags for map and copy

Constants

IOMMU_IOAS_MAP_FIXED_IOVA

If clear the kernel will compute an appropriate IOVA to place the mapping at

IOMMU_IOAS_MAP_WRITEABLE

DMA is allowed to write to this mapping

IOMMU_IOAS_MAP_READABLE

DMA is allowed to read from this mapping

struct iommu_ioas_map

ioctl(IOMMU_IOAS_MAP)

Definition:

struct iommu_ioas_map {
    __u32 size;
    __u32 flags;
    __u32 ioas_id;
    __u32 __reserved;
    __aligned_u64 user_va;
    __aligned_u64 length;
    __aligned_u64 iova;
};

Members

size

sizeof(struct iommu_ioas_map)

flags

Combination of enum iommufd_ioas_map_flags

ioas_id

IOAS ID to change the mapping of

__reserved

Must be 0

user_va

Userspace pointer to start mapping from

length

Number of bytes to map

iova

IOVA the mapping was placed at. If IOMMU_IOAS_MAP_FIXED_IOVA is set then this must be provided as input.

Description

Set an IOVA mapping from a user pointer. If FIXED_IOVA is specified then the mapping will be established at iova, otherwise a suitable location based on the reserved and allowed lists will be automatically selected and returned in iova.

If IOMMU_IOAS_MAP_FIXED_IOVA is specified then the iova range must currently be unused, existing IOVA cannot be replaced.

struct iommu_ioas_copy

ioctl(IOMMU_IOAS_COPY)

Definition:

struct iommu_ioas_copy {
    __u32 size;
    __u32 flags;
    __u32 dst_ioas_id;
    __u32 src_ioas_id;
    __aligned_u64 length;
    __aligned_u64 dst_iova;
    __aligned_u64 src_iova;
};

Members

size

sizeof(struct iommu_ioas_copy)

flags

Combination of enum iommufd_ioas_map_flags

dst_ioas_id

IOAS ID to change the mapping of

src_ioas_id

IOAS ID to copy from

length

Number of bytes to copy and map

dst_iova

IOVA the mapping was placed at. If IOMMU_IOAS_MAP_FIXED_IOVA is set then this must be provided as input.

src_iova

IOVA to start the copy

Description

Copy an already existing mapping from src_ioas_id and establish it in dst_ioas_id. The src iova/length must exactly match a range used with IOMMU_IOAS_MAP.

This may be used to efficiently clone a subset of an IOAS to another, or as a kind of 'cache' to speed up mapping. Copy has an efficiency advantage over establishing equivalent new mappings, as internal resources are shared, and the kernel will pin the user memory only once.

struct iommu_ioas_unmap

ioctl(IOMMU_IOAS_UNMAP)

Definition:

struct iommu_ioas_unmap {
    __u32 size;
    __u32 ioas_id;
    __aligned_u64 iova;
    __aligned_u64 length;
};

Members

size

sizeof(struct iommu_ioas_unmap)

ioas_id

IOAS ID to change the mapping of

iova

IOVA to start the unmapping at

length

Number of bytes to unmap, and return back the bytes unmapped

Description

Unmap an IOVA range. The iova/length must be a superset of a previously mapped range used with IOMMU_IOAS_MAP or IOMMU_IOAS_COPY. Splitting or truncating ranges is not allowed. The values 0 to U64_MAX will unmap everything.

enum iommufd_option

ioctl(IOMMU_OPTION_RLIMIT_MODE) and ioctl(IOMMU_OPTION_HUGE_PAGES)

Constants

IOMMU_OPTION_RLIMIT_MODE

Change how RLIMIT_MEMLOCK accounting works. The caller must have privilege to invoke this. Value 0 (default) is user based accouting, 1 uses process based accounting. Global option, object_id must be 0

IOMMU_OPTION_HUGE_PAGES

Value 1 (default) allows contiguous pages to be combined when generating iommu mappings. Value 0 disables combining, everything is mapped to PAGE_SIZE. This can be useful for benchmarking. This is a per-IOAS option, the object_id must be the IOAS ID.

enum iommufd_option_ops

ioctl(IOMMU_OPTION_OP_SET) and ioctl(IOMMU_OPTION_OP_GET)

Constants

IOMMU_OPTION_OP_SET

Set the option's value

IOMMU_OPTION_OP_GET

Get the option's value

struct iommu_option

iommu option multiplexer

Definition:

struct iommu_option {
    __u32 size;
    __u32 option_id;
    __u16 op;
    __u16 __reserved;
    __u32 object_id;
    __aligned_u64 val64;
};

Members

size

sizeof(struct iommu_option)

option_id

One of enum iommufd_option

op

One of enum iommufd_option_ops

__reserved

Must be 0

object_id

ID of the object if required

val64

Option value to set or value returned on get

Description

Change a simple option value. This multiplexor allows controlling options on objects. IOMMU_OPTION_OP_SET will load an option and IOMMU_OPTION_OP_GET will return the current value.

enum iommufd_vfio_ioas_op

IOMMU_VFIO_IOAS_* ioctls

Constants

IOMMU_VFIO_IOAS_GET

Get the current compatibility IOAS

IOMMU_VFIO_IOAS_SET

Change the current compatibility IOAS

IOMMU_VFIO_IOAS_CLEAR

Disable VFIO compatibility

struct iommu_vfio_ioas

ioctl(IOMMU_VFIO_IOAS)

Definition:

struct iommu_vfio_ioas {
    __u32 size;
    __u32 ioas_id;
    __u16 op;
    __u16 __reserved;
};

Members

size

sizeof(struct iommu_vfio_ioas)

ioas_id

For IOMMU_VFIO_IOAS_SET the input IOAS ID to set For IOMMU_VFIO_IOAS_GET will output the IOAS ID

op

One of enum iommufd_vfio_ioas_op

__reserved

Must be 0

Description

The VFIO compatibility support uses a single ioas because VFIO APIs do not support the ID field. Set or Get the IOAS that VFIO compatibility will use. When VFIO_GROUP_SET_CONTAINER is used on an iommufd it will get the compatibility ioas, either by taking what is already set, or auto creating one. From then on VFIO will continue to use that ioas and is not effected by this ioctl. SET or CLEAR does not destroy any auto-created IOAS.

IOMMUFD Kernel API

The IOMMUFD kAPI is device-centric with group-related tricks managed behind the scene. This allows the external drivers calling such kAPI to implement a simple device-centric uAPI for connecting its device to an iommufd, instead of explicitly imposing the group semantics in its uAPI as VFIO does.

struct iommufd_device *iommufd_device_bind(struct iommufd_ctx *ictx, struct device *dev, u32 *id)

Bind a physical device to an iommu fd

Parameters

struct iommufd_ctx *ictx

iommufd file descriptor

struct device *dev

Pointer to a physical device struct

u32 *id

Output ID number to return to userspace for this device

Description

A successful bind establishes an ownership over the device and returns struct iommufd_device pointer, otherwise returns error pointer.

A driver using this API must set driver_managed_dma and must not touch the device until this routine succeeds and establishes ownership.

Binding a PCI device places the entire RID under iommufd control.

The caller must undo this with iommufd_device_unbind()

void iommufd_device_unbind(struct iommufd_device *idev)

Undo iommufd_device_bind()

Parameters

struct iommufd_device *idev

Device returned by iommufd_device_bind()

Description

Release the device from iommufd control. The DMA ownership will return back to unowned with DMA controlled by the DMA API. This invalidates the iommufd_device pointer, other APIs that consume it must not be called concurrently.

int iommufd_device_attach(struct iommufd_device *idev, u32 *pt_id)

Connect a device from an iommu_domain

Parameters

struct iommufd_device *idev

device to attach

u32 *pt_id

Input a IOMMUFD_OBJ_IOAS, or IOMMUFD_OBJ_HW_PAGETABLE Output the IOMMUFD_OBJ_HW_PAGETABLE ID

Description

This connects the device to an iommu_domain, either automatically or manually selected. Once this completes the device could do DMA.

The caller should return the resulting pt_id back to userspace. This function is undone by calling iommufd_device_detach().

void iommufd_device_detach(struct iommufd_device *idev)

Disconnect a device to an iommu_domain

Parameters

struct iommufd_device *idev

device to detach

Description

Undo iommufd_device_attach(). This disconnects the idev from the previously attached pt_id. The device returns back to a blocked DMA translation.

struct iommufd_access *iommufd_access_create(struct iommufd_ctx *ictx, const struct iommufd_access_ops *ops, void *data, u32 *id)

Create an iommufd_access

Parameters

struct iommufd_ctx *ictx

iommufd file descriptor

const struct iommufd_access_ops *ops

Driver's ops to associate with the access

void *data

Opaque data to pass into ops functions

u32 *id

Output ID number to return to userspace for this access

Description

An iommufd_access allows a driver to read/write to the IOAS without using DMA. The underlying CPU memory can be accessed using the iommufd_access_pin_pages() or iommufd_access_rw() functions.

The provided ops are required to use iommufd_access_pin_pages().

void iommufd_access_destroy(struct iommufd_access *access)

Destroy an iommufd_access

Parameters

struct iommufd_access *access

The access to destroy

Description

The caller must stop using the access before destroying it.

void iommufd_access_unpin_pages(struct iommufd_access *access, unsigned long iova, unsigned long length)

Undo iommufd_access_pin_pages

Parameters

struct iommufd_access *access

IOAS access to act on

unsigned long iova

Starting IOVA

unsigned long length

Number of bytes to access

Description

Return the struct page's. The caller must stop accessing them before calling this. The iova/length must exactly match the one provided to access_pages.

int iommufd_access_pin_pages(struct iommufd_access *access, unsigned long iova, unsigned long length, struct page **out_pages, unsigned int flags)

Return a list of pages under the iova

Parameters

struct iommufd_access *access

IOAS access to act on

unsigned long iova

Starting IOVA

unsigned long length

Number of bytes to access

struct page **out_pages

Output page list

unsigned int flags

IOPMMUFD_ACCESS_RW_* flags

Description

Reads length bytes starting at iova and returns the struct page * pointers. These can be kmap'd by the caller for CPU access.

The caller must perform iommufd_access_unpin_pages() when done to balance this.

This API always requires a page aligned iova. This happens naturally if the ioas alignment is >= PAGE_SIZE and the iova is PAGE_SIZE aligned. However smaller alignments have corner cases where this API can fail on otherwise aligned iova.

int iommufd_access_rw(struct iommufd_access *access, unsigned long iova, void *data, size_t length, unsigned int flags)

Read or write data under the iova

Parameters

struct iommufd_access *access

IOAS access to act on

unsigned long iova

Starting IOVA

void *data

Kernel buffer to copy to/from

size_t length

Number of bytes to access

unsigned int flags

IOMMUFD_ACCESS_RW_* flags

Description

Copy kernel to/from data into the range given by IOVA/length. If flags indicates IOMMUFD_ACCESS_RW_KTHREAD then a large copy can be optimized by changing it into copy_to/from_user().

void iommufd_ctx_get(struct iommufd_ctx *ictx)

Get a context reference

Parameters

struct iommufd_ctx *ictx

Context to get

Description

The caller must already hold a valid reference to ictx.

struct iommufd_ctx *iommufd_ctx_from_file(struct file *file)

Acquires a reference to the iommufd context

Parameters

struct file *file

File to obtain the reference from

Description

Returns a pointer to the iommufd_ctx, otherwise ERR_PTR. The struct file remains owned by the caller and the caller must still do fput. On success the caller is responsible to call iommufd_ctx_put().

void iommufd_ctx_put(struct iommufd_ctx *ictx)

Put back a reference

Parameters

struct iommufd_ctx *ictx

Context to put back

VFIO and IOMMUFD

Connecting a VFIO device to iommufd can be done in two ways.

First is a VFIO compatible way by directly implementing the /dev/vfio/vfio container IOCTLs by mapping them into io_pagetable operations. Doing so allows the use of iommufd in legacy VFIO applications by symlinking /dev/vfio/vfio to /dev/iommufd or extending VFIO to SET_CONTAINER using an iommufd instead of a container fd.

The second approach directly extends VFIO to support a new set of device-centric user API based on aforementioned IOMMUFD kernel API. It requires userspace change but better matches the IOMMUFD API semantics and easier to support new iommufd features when comparing it to the first approach.

Currently both approaches are still work-in-progress.

There are still a few gaps to be resolved to catch up with VFIO type1, as documented in iommufd_vfio_check_extension().

Future TODOs

Currently IOMMUFD supports only kernel-managed I/O page table, similar to VFIO type1. New features on the radar include:

  • Binding iommu_domain's to PASID/SSID

  • Userspace page tables, for ARM, x86 and S390

  • Kernel bypass'd invalidation of user page tables

  • Re-use of the KVM page table in the IOMMU

  • Dirty page tracking in the IOMMU

  • Runtime Increase/Decrease of IOPTE size

  • PRI support with faults resolved in userspace