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Manual Pages  — IOAT


I/OAT – Intel I/O Acceleration Technology



To compile this driver into your kernel, place the following line in your kernel configuration file: device ioat

Or, to load the driver as a module at boot, place the following line in loader.conf(5):


In loader.conf(5):


In loader.conf(5) or sysctl.conf(5):

hw.ioat.enable_ioat_test=0 hw.ioat.debug_level=0 (only critical errors; maximum of 3)

typedef void
(*bus_dmaengine_callback_t)(void *arg, int error);

ioat_get_dmaengine(uint32_t channel_index);

ioat_put_dmaengine(bus_dmaengine_t dmaengine);

ioat_get_hwversion(bus_dmaengine_t dmaengine);

ioat_get_max_io_size(bus_dmaengine_t dmaengine);

ioat_set_interrupt_coalesce(bus_dmaengine_t dmaengine, uint16_t delay);

ioat_get_max_coalesce_period(bus_dmaengine_t dmaengine);

ioat_acquire(bus_dmaengine_t dmaengine);

ioat_acquire_reserve(bus_dmaengine_t dmaengine, uint32_t n, int mflags);

ioat_release(bus_dmaengine_t dmaengine);

struct bus_dmadesc *
ioat_copy(bus_dmaengine_t dmaengine, bus_addr_t dst, bus_addr_t src, bus_size_t len, bus_dmaengine_callback_t callback_fn, void *callback_arg, uint32_t flags);

struct bus_dmadesc *
ioat_copy_8k_aligned(bus_dmaengine_t dmaengine, bus_addr_t dst1, bus_addr_t dst2, bus_addr_t src1, bus_addr_t src2, bus_dmaengine_callback_t callback_fn, void *callback_arg, uint32_t flags);

struct bus_dmadesc *
ioat_copy_crc(bus_dmaengine_t dmaengine, bus_addr_t dst, bus_addr_t src, bus_size_t len, uint32_t *initialseed, bus_addr_t crcptr, bus_dmaengine_callback_t callback_fn, void *callback_arg, uint32_t flags);

struct bus_dmadesc *
ioat_crc(bus_dmaengine_t dmaengine, bus_addr_t src, bus_size_t len, uint32_t *initialseed, bus_addr_t crcptr, bus_dmaengine_callback_t callback_fn, void *callback_arg, uint32_t flags);

struct bus_dmadesc *
ioat_blockfill(bus_dmaengine_t dmaengine, bus_addr_t dst, uint64_t fillpattern, bus_size_t len, bus_dmaengine_callback_t callback_fn, void *callback_arg, uint32_t flags);

struct bus_dmadesc *
ioat_null(bus_dmaengine_t dmaengine, bus_dmaengine_callback_t callback_fn, void *callback_arg, uint32_t flags);


The I/OAT driver provides a kernel API to a variety of DMA engines on some Intel server platforms.

There is a number of DMA channels per CPU package. (Typically 4 or 8.) Each may be used independently. Operations on a single channel proceed sequentially.

Blockfill operations can be used to write a 64-bit pattern to memory.

Copy operations can be used to offload memory copies to the DMA engines.

Null operations do nothing, but may be used to test the interrupt and callback mechanism.

All operations can optionally trigger an interrupt at completion with the DMA_INT_EN flag. For example, a user might submit multiple operations to the same channel and only enable an interrupt and callback for the last operation.

The hardware can delay and coalesce interrupts on a given channel for a configurable period of time, in microseconds. This may be desired to reduce the processing and interrupt overhead per descriptor, especially for workflows consisting of many small operations. Software can control this on a per-channel basis with the ioat_set_interrupt_coalesce() API. The ioat_get_max_coalesce_period() API can be used to determine the maximum coalescing period supported by the hardware, in microseconds. Current platforms support up to a 16.383 millisecond coalescing period. Optimal configuration will vary by workflow and desired operation latency.

All operations are safe to use in a non-blocking context with the DMA_NO_WAIT flag. (Of course, allocations may fail and operations requested with DMA_NO_WAIT may return NULL.)

Operations that depend on the result of prior operations should use DMA_FENCE. For example, such a scenario can happen when two related DMA operations are queued. First, a DMA copy to one location (A), followed directly by a DMA copy from A to B. In this scenario, some classes of I/OAT hardware may prefetch A for the second operation before it is written by the first operation. To avoid reading a stale value in sequences of dependent operations, use DMA_FENCE.

All operations, as well as ioat_get_dmaengine(), can return NULL in special circumstances. For example, if the I/OAT driver is being unloaded, or the administrator has induced a hardware reset, or a usage error has resulted in a hardware error state that needs to be recovered from.

It is invalid to attempt to submit new DMA operations in a bus_dmaengine_callback_t context.

The CRC operations have three distinct modes. The default mode is to accumulate. By accumulating over multiple descriptors, a user may gather a CRC over several chunks of memory and only write out the result once.

The DMA_CRC_STORE flag causes the operation to emit the CRC32C result. If DMA_CRC_INLINE is set, the result is written inline with the destination data (or source in ioat_crc() mode). If DMA_CRC_INLINE is not set, the result is written to the provided crcptr.

Similarly, the DMA_CRC_TEST flag causes the operation to compare the CRC32C result to an existing checksum. If DMA_CRC_INLINE is set, the result is compared against the inline four bytes trailing the source data. If it is not set, the result is compared against the value pointed to by crcptr.

ioat_copy_crc() calculates a CRC32C while copying data. ioat_crc() only computes a CRC32C of some data. If the initialseed argument to either routine is non-NULL, the CRC32C engine is initialized with the value it points to.


A typical user will lookup the DMA engine object for a given channel with ioat_get_dmaengine(). When the user wants to offload a copy, they will first ioat_acquire() the bus_dmaengine_t object for exclusive access to enqueue operations on that channel. Optionally, the user can reserve space by using ioat_acquire_reserve() instead. If ioat_acquire_reserve() succeeds, there is guaranteed to be room for N new operations in the internal ring buffer.

Then, they will submit one or more operations using ioat_blockfill(), ioat_copy(), ioat_copy_8k_aligned(), ioat_copy_crc(), ioat_crc(), or ioat_null(). After queuing one or more individual DMA operations, they will ioat_release() the bus_dmaengine_t to drop their exclusive access to the channel. The routine they provided for the callback_fn argument will be invoked with the provided callback_arg when the operation is complete. When they are finished with the bus_dmaengine_t, the user should ioat_put_dmaengine().

Users MUST NOT block between ioat_acquire() and ioat_release(). Users SHOULD NOT hold bus_dmaengine_t references for a very long time to enable fault recovery and kernel module unload.

For an example of usage, see src/sys/dev/ioat/ioat_test.c.


/dev/ioat_test test device for ioatcontrol(8)




The I/OAT driver first appeared in FreeBSD 11.0 .


The I/OAT driver was developed by Jim Harris <Mt jimharris@FreeBSD.org>, Carl Delsey <Mt carl.r.delsey@intel.com>, and Conrad Meyer <Mt cem@FreeBSD.org>. This manual page was written by Conrad Meyer <Mt cem@FreeBSD.org>.


Copy operation takes bus addresses as parameters, not virtual addresses.

Buffers for individual copy operations must be physically contiguous.

Copies larger than max transfer size (1MB, but may vary by hardware) are not supported. Future versions will likely support this by breaking up the transfer into smaller sizes.


The I/OAT driver only supports blockfill, copy, and null operations at this time. The driver does not yet support advanced DMA modes, such as XOR, that some I/OAT devices support.

IOAT (4) May 3, 2016

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