CopyFromModeNonTemporal function (usermode_accessors.h)

The CopyFromModeNonTemporal function safely copies data from either kernel or user-mode memory to kernel memory using non-temporal instructions based on the specified processor mode.

Syntax

void CopyFromModeNonTemporal(
  VOID            *Destination,
  const VOID      *Source,
  SIZE_T          Length,
  KPROCESSOR_MODE Mode
);

Parameters

Destination

[out] A pointer to the kernel memory ___location where the data will be copied.

Source

[in] A pointer to the memory ___location from which to copy the data.

Length

[in] The number of bytes to copy.

Mode

[in] The processor mode that determines how the memory access is performed. Mode can be one of the following values.

Return value

None

Remarks

This function provides a safe way to copy data from either kernel or user-mode memory to kernel memory using non-temporal (streaming) instructions, with the copy mechanism determined by the specified processor mode. This allows for flexible memory operations that can adapt to different execution contexts while optimizing cache performance for large data transfers.

When Mode is KernelMode:

• The function performs a volatile copy using memory_order_relaxed semantics with non-temporal instructions.

• The function isn't recognized as a compiler intrinsic so the compiler will never optimize away the call (either entirely or replace the call with an equivalent sequence of instructions).

• When the call returns, the data has been copied from Source to Destination. This function's memory accesses to the Source and Destination will only be performed within the function (for example, the compiler can't move memory accesses out of this function).

• The function might perform unaligned memory accesses if the platform allows for it.

• The function might access memory locations more than once as part of its copy operation.

• The function doesn't support copy operations when Source and Destination overlap each other.

• The function uses non-temporal instructions that can improve performance for large data transfers by bypassing the processor cache, reducing cache pollution.

This function is particularly useful when copying large amounts of data that are unlikely to be accessed again soon, as it avoids evicting other useful data from the cache.

This function will never be optimized away by the compiler, nor will the compiler create additional accesses to this memory ___location before the function is called or after the function returns (unless the source code explicitly performs these accesses).

This function works on all versions of Windows, not just the latest. You need to consume the latest WDK to get the function declaration from the usermode_accessors.h header. You also need the library (umaccess.lib) from the latest WDK. However, the resulting driver will run fine on older versions of Windows.

Requirements

See also

CopyFromMode

CopyFromUserNonTemporal

CopyToModeNonTemporal