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 v1: * Minor rewordings and typo fixes. * Fix typo'd #define names. * Add type annotation for PR_SVE_SET_VL arg2. * Clarify return value semantics of PR_SVE_SET_VL * Add note to say that the args for PR_SVE_GET_VL are ignored. * Note for PR_SVE_SET_VL that the PR_SVE_VL_LEN_MASK field specifies an upper bound on what vector length to set, not an exact value. * Rework PR_SVE_SET_VL arg2 description to enumerate all possible flag combinations rather than describing the flags independently. Coming up with a clear description of each flag that is independent of the description of the other flag turns out to be hard. * In lieu of having a separate man page to cross reference for detailed guidance, cross-reference the kernel documentation. * Avoid confusing cross-reference to PR_SVE_SET_VL when describing the return value of PR_SVE_GET_VL. * Clarify error conditions for PR_SVE_SET_VL and PR_SVE_GET_VL, and move detail to the individual prctl descriptions to keep related content close together while minimising duplication. * Add safety warning. This is deliberately vague, pending ongoing discussions with libc folks. --- man2/prctl.2 | 160 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 160 insertions(+) diff --git a/man2/prctl.2 b/man2/prctl.2 index cab9915..91df7c8 100644 --- a/man2/prctl.2 +++ b/man2/prctl.2 @@ -1291,6 +1291,148 @@ 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, +.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, +which may differ from the current configuration if +.B PR_SVE_SET_VL_ONEXEC +was specified. +The value is encoded in the same way as the return value of +.BR PR_SVE_GET_VL . +.IP +The configuration (including any pending deferred change) +is inherited across +.BR fork (2) +and +.BR clone (2). +.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. +.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_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 @@ -1534,6 +1676,8 @@ 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 , @@ -1817,6 +1961,22 @@ and unused arguments to .BR prctl () are not 0. .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
