PR_SVE_SET_VL(2const)

NAME
PR_SVE_SET_VL - set the thread's SVE vector length

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>

int prctl(PR_SVE_SET_VL, unsigned long val);

DESCRIPTION
Configure the thread's SVE vector length, as specified by val.

The bits of val corresponding to PR_SVE_VL_LEN_MASK must be set to the desired vector length in bytes. This is interpreted as an upper bound: the kernel will select the greatest available vector length that does not exceed the value specified. In particular, specifying SVE_VL_MAX (defined in <asm/sigcontext.h>) for the PR_SVE_VL_LEN_MASK bits requests the maximum supported vector length.

In addition, the other bits of val must be set to one of the following combinations of flags:

0L  Perform the change immediately. At the next execve(2) in the thread, the vector length will be reset to the value configured in /proc/sys/abi/sve_default_vector_length.

PR_SVE_VL_INHERIT
    Perform the change immediately. Subsequent execve(2) calls will preserve the new vector length.

PR_SVE_SET_VL_ONEXEC
    Defer the change, so that it is performed at the next execve(2) in the thread. Further execve(2) calls will reset the vector length to the value configured in /proc/sys/abi/sve_default_vector_length.

PR_SVE_SET_VL_ONEXEC | PR_SVE_VL_INHERIT
    Defer the change, so that it is performed at the next execve(2) in the thread. Further execve(2) calls will preserve the new vector length.

In all cases, any previously pending deferred change is canceled.

On success, a nonnegative value is returned that describes the selected configuration. If PR_SVE_SET_VL_ONEXEC was included in val, then the configuration described by the return value will take effect at the next execve(2). Otherwise, the configuration is already in effect when the PR_SVE_SET_VL call returns. In either case, the value is encoded in the same way as the return value of PR_SVE_GET_VL. Note that there is no explicit flag in the return value corresponding to PR_SVE_SET_VL_ONEXEC.

The configuration (including any pending deferred change) is inherited across fork(2) and clone(2).

RETURN VALUE
On success, PR_SVE_SET_VL returns the nonnegative value described above. On error, -1 is returned, and errno is set to indicate the error.

ERRORS

EINVAL
    SVE is not available on this platform.

EINVAL
    The value in the bits of val corresponding to PR_SVE_VL_LEN_MASK is outside the range [SVE_VL_MIN, SVE_VL_MAX] or is not a multiple of 16.

EINVAL
    The other bits of val are invalid or unsupported.

FILES

/proc/sys/abi/sve_default_vector_length

STANDARDS
Linux. arm64 only.

HISTORY
Linux 4.15 (arm64).

CAVEATS
Because the compiler or run-time environment may be using SVE, using this call without the PR_SVE_SET_VL_ONEXEC flag may crash the calling process. The conditions for using it safely are complex and system-dependent. Don't use it unless you really know what you are doing.

SEE ALSO
prctl(2), PR_SVE_GET_VL(2const)

For more information, see the kernel source file Documentation/arm64/sve.rst (or Documentation/arm64/sve.txt before Linux 5.3).