Add documentation for the the PR_SVE_SET_VL and PR_SVE_GET_VL prctls added in Linux 4.15 for arm64. Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> --- Since v2: * Clarify that the arg2 enumeration for PR_SVE_SET_VL applies to the bith _other than_ the PR_SVE_VL_LEN_MASK bits, rather than to all the bits. * Clarify return value semantics for PR_SVE_SET_VL, to highlight that there is no PR_SVE_SET_VL_ONEXEC in the return value and refer to PR_SVE_GET_VL for the rest of the definition. Also clarify when the vector length given in the return value actually takes effect. * Reorder some documentation cross-references to avoid ambiguity about what they apply to. --- man2/prctl.2 | 170 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 170 insertions(+) diff --git a/man2/prctl.2 b/man2/prctl.2 index 88b791b..46ea9d2 100644 --- a/man2/prctl.2 +++ b/man2/prctl.2 @@ -1370,6 +1370,158 @@ call failing with the error .BR ENXIO . For further details, see the kernel source file .IR Documentation/admin\-guide/kernel\-parameters.txt . +.\" prctl PR_SVE_SET_VL +.\" commit 2d2123bc7c7f843aa9db87720de159a049839862 +.\" linux-5.6/Documentation/arm64/sve.rst +.TP +.BR PR_SVE_SET_VL " (since Linux 4.15, only on arm64)" +Configure the thread's SVE vector length, +as specified by +.IR "(int) arg2" . +Arguments +.IR arg3 ", " arg4 " and " arg5 +are ignored. +.IP +The bits of +.I arg2 +corresponding to +.B 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 +.B SVE_VL_MAX +(defined in +.I <asm/sigcontext.h>) +for the +.B PR_SVE_VL_LEN_MASK +bits requests the maximum supported vector length. +.IP +In addition, the other bits of +.I arg2 +must be set to one of the following combinations of flags: +.RS +.TP +.B 0 +Perform the change immediately. +At the next +.BR execve (2) +in the thread, +the vector length will be reset to the value configured in +.IR /proc/sys/abi/sve_default_vector_length . +.TP +.B PR_SVE_VL_INHERIT +Perform the change immediately. +Subsequent +.BR execve (2) +calls will preserve the new vector length. +.TP +.B PR_SVE_SET_VL_ONEXEC +Defer the change, so that it is performed at the next +.BR execve (2) +in the thread. +Further +.BR execve (2) +calls will reset the vector length to the value configured in +.IR /proc/sys/abi/sve_default_vector_length . +.TP +.B "PR_SVE_SET_VL_ONEXEC | PR_SVE_VL_INHERIT" +Defer the change, so that it is performed at the next +.BR execve (2) +in the thread. +Further +.BR execve (2) +calls will preserve the new vector length. +.RE +.IP +In all cases, +any previously pending deferred change is canceled. +.IP +The call fails with error +.B EINVAL +if SVE is not supported on the platform, if +.I arg2 +is unrecognized or invalid, or the value in the bits of +.I arg2 +corresponding to +.B PR_SVE_VL_LEN_MASK +is outside the range +.BR SVE_VL_MIN .. SVE_VL_MAX +or is not a multiple of 16. +.IP +On success, +a nonnegative value is returned that describes the +.I selected +configuration. +If +.B PR_SVE_SET_VL_ONEXEC +was included in +.IR arg2 , +then the configuration described by the return value +will take effect at the next +.BR execve (). +Otherwise, the configuration is already in effect when the +.B PR_SVE_SET_VL +call returns. +In either case, the value is encoded in the same way as the return value of +.BR PR_SVE_GET_VL . +Note that there is no explicit flag in the return value +corresponding to +.BR PR_SVE_SET_VL_ONEXEC . +.IP +The configuration (including any pending deferred change) +is inherited across +.BR fork (2) +and +.BR clone (2). +.IP +For more information, see the kernel source file +.I Documentation/arm64/sve.rst +.\"commit b693d0b372afb39432e1c49ad7b3454855bc6bed +(or +.I Documentation/arm64/sve.txt +before Linux 5.3). +.IP +.B Warning: +Because the compiler or run-time environment +may be using SVE, using this call without the +.B 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. +.\" prctl PR_SVE_GET_VL +.TP +.BR PR_SVE_GET_VL " (since Linux 4.15, only on arm64)" +Get the thread's current SVE vector length configuration. +.IP +Arguments +.IR arg2 ", " arg3 ", " arg4 " and " arg5 +are ignored. +.IP +Providing that the kernel and platform support SVE +this operation always succeeds, +returning a nonnegative value that describes the +.I current +configuration. +The bits corresponding to +.B PR_SVE_VL_LEN_MASK +contain the currently configured vector length in bytes. +The bit corresponding to +.B PR_SVE_VL_INHERIT +indicates whether the vector length will be inherited +across +.BR execve (2). +.IP +Note that there is no way to determine whether there is +a pending vector length change that has not yet taken effect. +.IP +For more information, see the kernel source file +.I Documentation/arm64/sve.rst +.\"commit b693d0b372afb39432e1c49ad7b3454855bc6bed +(or +.I Documentation/arm64/sve.txt +before Linux 5.3). .\" .\" prctl PR_TASK_PERF_EVENTS_DISABLE .TP @@ -1613,6 +1765,8 @@ On success, .BR PR_GET_NO_NEW_PRIVS , .BR PR_GET_SECUREBITS , .BR PR_GET_SPECULATION_CTRL , +.BR PR_SVE_GET_VL , +.BR PR_SVE_SET_VL , .BR PR_GET_THP_DISABLE , .BR PR_GET_TIMING , .BR PR_GET_TIMERSLACK , @@ -1904,6 +2058,22 @@ See the description of .B PR_PAC_RESET_KEYS above for details. .TP +.B EINVAL +.I option +is +.B PR_SVE_SET_VL +and the arguments are invalid or unsupported, +or SVE is not available on this platform. +See the description of +.B PR_SVE_SET_VL +above for details. +.TP +.B EINVAL +.I option +is +.B PR_SVE_GET_VL +and SVE is not available on this platform. +.TP .B ENODEV .I option was -- 2.1.4
