Hi Dave, On Tue, May 12, 2020 at 05:36:58PM +0100, Dave Martin wrote: > diff --git a/man2/prctl.2 b/man2/prctl.2 > index 7f511d2..dd16227 100644 > --- a/man2/prctl.2 > +++ b/man2/prctl.2 > @@ -1291,6 +1291,104 @@ 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 arg2 . > +Arguments > +.IR arg3 ", " arg4 " and " arg5 > +are ignored. Bugger, did we forget to force these to zero? I guess we should write the man-page first next time :( > +.IP > +The bits of > +.I arg2 > +corresponding to > +.B SVE_VL_LEN_MASK PR_SVE_LEN_MASK > +must be set to the desired vector length in bytes. > +In addition, > +.I arg2 > +may include zero or more of the following flags: > +.RS > +.TP > +.B PR_SVE_VL_INHERIT > +Inherit the configured vector length across > +.BR execve (2). > +.TP > +.B PR_SVE_SET_VL_ONEXEC > +Defer the change until the next > +.BR execve (2) > +in this thread. (aside, it's weird that we didn't allocate (1<<16) for one of these flags) > +If > +.B PR_SVE_VL_INHERIT > +is also included in > +.IR arg2 , > +it takes effect > +.I after > +this deferred change. I find this a bit hard to follow, since it's not clear to me whether the INHERIT flag is effectively set before or after the next execve(). In other words, if both PR_SVE_SET_VL_ONEXEC and PR_SVE_VL_INHERIT are specified, is the vector length preserved or reset on the next execve()? > +.RE > +.IP > +On success, the vector length and flags are set as requested, > +and any deferred change that was pending immediately before the > +.B PR_SVE_SET_VL > +call is canceled. Huh, turns out 'canceled' is a valid US spelling. Fair enough, but it looks wrong to me ;) > +If > +.B PR_SVE_SET_VL_ONEXEC > +was included in > +.IR arg2 , > +the returned value describes the configuration > +scheduled to take effect at the next > +.BR execve (2). "describes the configuration" how? > +Otherwise, the effect is immediate and > +the returned value describes the new configuration. > +The returned value is encoded in the same way as the return value of > +.BR PR_SVE_GET_VL . Aha. Maybe move this bit up slightly? > +.IP > +If neither of the above flags is included in are included > +.IR arg2 , > +a subsequent > +.BR execve (2) > +resets the vector length to the default value configured in > +.IR /proc/sys/abi/sve_default_vector_length . > +.IP > +The actual vector length configured by this operation > +is the greatest vector length supported by the platform > +that does not exceed > +.I arg2 > +& > +.BR PR_SVE_VL_LEN_MASK . > +.IP > +The configuration (including any pending deferred change) > +is inherited across > +.BR fork (2) > +and > +.BR clone (2). > +.\" 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, > +as configured by > +.BR PR_SVE_SET_VL . It doesn't *have* to be configured by PR_SVE_SET_VL though, right? > +.IP > +If successful, the return value describes the > +.I current > +configuration. (aside: prctl() returns int, so we can't ever allocate past bit 30 in arg2. Might be worth a note somewhere in the kernel). > +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 determine whether there is to determine > +a pending vector length change that has not yet taken effect. > +.IP > +Providing that the kernel and platform support SVE, > +this operation always succeeds. > .\" > .\" prctl PR_TASK_PERF_EVENTS_DISABLE > .TP > @@ -1534,6 +1632,8 @@ 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 +1917,18 @@ and unused arguments to > .BR prctl () > are not 0. > .TP > +.B EINVAL > +.I option > +was > +.B PR_SVE_SET_VL > +and > +.I arg2 > +contains invalid flags, or > +.I arg2 > +& > +.B SVE_VL_LEN_MASK > +is not a valid vector length. > +.TP PR_SVE_GET_VL can return -EINVAL if SVE is not supported. Will
