Re: [PATCH -next v21 25/27] riscv: Add documentation for Vector

Rémi Denis-Courmont
2023-06-12T14:40:52Z

Nits...

Le maanantaina 5. kesäkuuta 2023, 14.07.22 EEST Andy Chiu a écrit :
> This patch add a brief documentation of the userspace interface in
> regard to the RISC-V Vector extension.
>
> Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
> Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
> Reviewed-by: Vincent Chen <vincent.chen@sifive.com>
> Co-developed-by: Bagas Sanjaya <bagasdotme@gmail.com>
> Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
> ---
> Changelog v21:
> - add usage guideline into doc (Rémi)
> Changelog v20:
> - Drop bit-field repressentation and typos (Björn)
> - Fix document styling (Bagas)
> ---
> Documentation/riscv/index.rst | 1 +
> Documentation/riscv/vector.rst | 132 +++++++++++++++++++++++++++++++++
> 2 files changed, 133 insertions(+)
> create mode 100644 Documentation/riscv/vector.rst
>
> diff --git a/Documentation/riscv/index.rst b/Documentation/riscv/index.rst
> index 175a91db0200..95cf9c1e1da1 100644
> --- a/Documentation/riscv/index.rst
> +++ b/Documentation/riscv/index.rst
> @@ -10,6 +10,7 @@ RISC-V architecture
> hwprobe
> patch-acceptance
> uabi
> + vector
>
> features
>
> diff --git a/Documentation/riscv/vector.rst b/Documentation/riscv/vector.rst
> new file mode 100644
> index 000000000000..48f189d79e41
> --- /dev/null
> +++ b/Documentation/riscv/vector.rst
> @@ -0,0 +1,132 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=========================================
> +Vector Extension Support for RISC-V Linux
> +=========================================
> +
> +This document briefly outlines the interface provided to userspace by Linux
> in +order to support the use of the RISC-V Vector Extension.
> +
> +1. prctl() Interface
> +---------------------
> +
> +Two new prctl() calls are added to allow programs to manage the enablement
> +status for the use of Vector in userspace. The intended usage guideline for
> +these interfaces is to give init systems a way to modify the availability
> of V +for processes running under its domain.

Nit: their domain

> Calling thess interfaces is

those interfaces

> not +recommended in libraries routines because libraries should not
> override policies +configured from the parant process. Also, users must

parent

> noted that these interfaces

note

> +are not portable to non-Linux, nor non-RISC-V
> environments, so it is discourage +to use in a portable code.

"so their use in portable code is discouraged."

> To get the
> availability of V in an ELF program, +please read
> :c:macro:`COMPAT_HWCAP_ISA_V` bit of :c:macro:`ELF_HWCAP` in the +auxiliary
> vector.
> +
> +* prctl(PR_RISCV_V_SET_CONTROL, unsigned long arg)
> +
> + Sets the Vector enablement status of the calling thread, where the
> control + argument consists of two 2-bit enablement statuses and a bit
> for inheritance + mode. Other threads of the calling process are
> unaffected.
> +
> + Enablement status is a tri-state value each occupying 2-bit of space in
> + the control argument:
> +
> + * :c:macro:`PR_RISCV_V_VSTATE_CTRL_DEFAULT`: Use the system-wide
> default + enablement status on execve(). The system-wide default
> setting can be + controlled via sysctl interface (see sysctl section
> below). +
> + * :c:macro:`PR_RISCV_V_VSTATE_CTRL_ON`: Allow Vector to be run for the
> + thread.
> +
> + * :c:macro:`PR_RISCV_V_VSTATE_CTRL_OFF`: Disallow Vector. Executing
> Vector + instructions under such condition will trap and casuse the
> termination of the thread.

cause

> +
> + arg: The control argument is a 5-bit value consisting of 3 parts, and
> + accessed by 3 masks respectively.
> +
> + The 3 masks, PR_RISCV_V_VSTATE_CTRL_CUR_MASK,
> + PR_RISCV_V_VSTATE_CTRL_NEXT_MASK, and PR_RISCV_V_VSTATE_CTRL_INHERIT
> + represents bit[1:0], bit[3:2], and bit[4]. bit[1:0] accounts for the
> + enablement status of current thread, and the setting at bit[3:2] takes
> place + at next execve(). bit[4] defines the inheritance mode of the
> setting in + bit[3:2].
> +
> + * :c:macro:`PR_RISCV_V_VSTATE_CTRL_CUR_MASK`: bit[1:0]: Account for
> the + Vector enablement status for the calling thread. The calling
> thread is + not able to turn off Vector once it has been enabled.
> The prctl() call + fails with EPERM if the value in this mask is
> PR_RISCV_V_VSTATE_CTRL_OFF + but the current enablement status is
> not off. Setting
> + PR_RISCV_V_VSTATE_CTRL_DEFAULT here takes no effect but to set
> back + the original enablement status.
> +
> + * :c:macro:`PR_RISCV_V_VSTATE_CTRL_NEXT_MASK`: bit[3:2]: Account
> for the + Vector enablement setting for the calling thread at the
> next execve() + system call. If PR_RISCV_V_VSTATE_CTRL_DEFAULT is
> used in this mask, + then the enablement status will be decided by
> the system-wide + enablement status when execve() happen.
> +
> + * :c:macro:`PR_RISCV_V_VSTATE_CTRL_INHERIT`: bit[4]: the
> inheritance + mode for the setting at
> PR_RISCV_V_VSTATE_CTRL_NEXT_MASK. If the bit + is set then the
> following execve() will not clear the setting in both +
> PR_RISCV_V_VSTATE_CTRL_NEXT_MASK and PR_RISCV_V_VSTATE_CTRL_INHERIT. +
> This setting persists across changes in the system-wide default value.
> +
> + Return value:
> + * 0 on success;
> + * EINVAL: Vector not supported, invalid enablement status for
> current or + next mask;
> + * EPERM: Turning off Vector in PR_RISCV_V_VSTATE_CTRL_CUR_MASK if
> Vector + was enabled for the calling thread.
> +
> + On success:
> + * A valid setting for PR_RISCV_V_VSTATE_CTRL_CUR_MASK takes place
> + immediately. The enablement status specified in
> + PR_RISCV_V_VSTATE_CTRL_NEXT_MASK happens at the next execve()
> call, or + all following execve() calls if
> PR_RISCV_V_VSTATE_CTRL_INHERIT bit is + set.
> + * Every successful call overwrites a previous setting for the
> calling + thread.
> +
> +* prctl(PR_RISCV_V_GET_CONTROL)
> +
> + Gets the same Vector enablement status for the calling thread. Setting
> for + next execve() call and the inheritance bit are all OR-ed together.
> +
> + Note that ELF programs are able to get the availability of V for itself
> by + reading :c:macro:`COMPAT_HWCAP_ISA_V` bit of :c:macro:`ELF_HWCAP`
> in the + auxiliary vector.
> +
> + Return value:
> + * a nonnegative value on success;
> + * EINVAL: Vector not supported.
> +
> +2. System runtime configuration (sysctl)
> +-----------------------------------------
> +
> +To mitigate the ABI impact of expansion of the signal stack, a
> +policy mechanism is provided to the administrators, distro maintainers, and
> +developers to control the default Vector enablement status for userspace
> +processes in form of sysctl knob:
> +
> +* /proc/sys/abi/riscv_v_default_allow
> +
> + Writing the text representation of 0 or 1 to this file sets the default
> + system enablement status for new starting userspace programs. Valid
> values + are:
> +
> + * 0: Do not allow Vector code to be executed as the default for new
> processes. + * 1: Allow Vector code to be executed as the default for
> new processes. +
> + Reading this file returns the current system default enablement status.
> +
> + At every execve() call, a new enablement status of the new process is
> set to + the system default, unless:
> +
> + * PR_RISCV_V_VSTATE_CTRL_INHERIT is set for the calling process, and
> the + setting in PR_RISCV_V_VSTATE_CTRL_NEXT_MASK is not
> + PR_RISCV_V_VSTATE_CTRL_DEFAULT. Or,
> +
> + * The setting in PR_RISCV_V_VSTATE_CTRL_NEXT_MASK is not
> + PR_RISCV_V_VSTATE_CTRL_DEFAULT.
> +
> + Modifying the system default enablement status does not affect the
> enablement + status of any existing process of thread that do not make
> an execve() call.

--
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