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vmo_set_cache_policy.md

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zx_vmo_set_cache_policy

SUMMARY

Set the caching policy for pages held by a VMO.

DECLARATION

#include <zircon/syscalls.h>

zx_status_t zx_vmo_set_cache_policy(zx_handle_t handle, uint32_t cache_policy);

DESCRIPTION

zx_vmo_set_cache_policy() sets caching policy for a virtual memory object (VMO). Generally used on VMOs that point directly at physical memory. Such VMOs are generally only handed to userspace via bus protocol interfaces, so this syscall will typically only be used by drivers dealing with device memory. This call can also be used on a regular memory backed VMO with similar limitations and uses.

A handle must have the ZX_RIGHT_MAP right for this call to be permitted. Additionally, the VMO must not presently be mapped by any process, have any children, be a child itself, or have any pinned pages. Having committed pages is only allowed if transitioning from a cached state, and the pages will be cleaned and invalidated.

cache_policy cache flags to use:

  • ZX_CACHE_POLICY_CACHED - Use hardware caching. On Aarch64 this corresponds to the Normal Memory, Outer Write-back non-transient Read and Write allocate, Inner Write-back non-transient Read and Write allocate memory attributes

  • ZX_CACHE_POLICY_UNCACHED - Disable caching. On Aarch64 this corresponds to the Device-nGnRnE memory attributes.

  • ZX_CACHE_POLICY_UNCACHED_DEVICE - Disable cache and treat as device memory. This is architecture dependent and may be equivalent to ZX_CACHE_POLICY_UNCACHED on some architectures. On Aarch64 this corresponds to the Device-nGnRE memory attributes.

  • ZX_CACHE_POLICY_WRITE_COMBINING - Uncached with write combining. On Aarch64 this corresponds to the Normal memory, uncached memory attributes.

RIGHTS

handle must be of type ZX_OBJ_TYPE_VMO and have ZX_RIGHT_MAP.

RETURN VALUE

zx_vmo_set_cache_policy() returns ZX_OK on success. In the event of failure, a negative error value is returned.

ERRORS

ZX_ERR_ACCESS_DENIED Cache policy has been configured for this VMO already and may not be changed, or handle lacks the ZX_RIGHT_MAP right.

ZX_ERR_BAD_HANDLE handle is not a valid handle.

ZX_ERR_INVALID_ARGS cache_policy contains flags outside of the ones listed above, or cache_policy contains an invalid mix of cache policy flags.

ZX_ERR_NOT_SUPPORTED The VMO handle corresponds to is not one holding physical memory.

ZX_ERR_BAD_STATE Cache policy cannot be changed because the VMO is presently mapped, has children, is a child itself, has pinned pages, or has committed pages and the VMO is currently not cached.

SEE ALSO