Re: [PATCH v2 4/6] prctl.2: Add SVE prctls (arm64)

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Dave,

I've not applied this patch yet, in case you want to make
some changes in response to Will's comments.

I think all of the rest of the patches in the series are now
applied (and pushed to master).

Thanks,

Michael

On 5/27/20 11:17 PM, Dave Martin wrote:
> 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@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> 
> ---
> 
> 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
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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