Ubuntu Manpage: PR_SVE_SET_VL - set the thread's SVE vector length

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.