riscv: Add documentation for Vector

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>
Link: https://lore.kernel.org/r/20230605110724.21391-26-andy.chiu@sifive.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/riscv/index.rst

@@ -10,6 +10,7 @@ RISC-V architecture
     hwprobe
     patch-acceptance
     uabi
+    vector
 
     features
 

Documentation/riscv/vector.rst

+.. 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. Calling thess interfaces is not
+recommended in libraries routines because libraries should not override policies
+configured from the parant process. Also, users must noted that these interfaces
+are not portable to non-Linux, nor non-RISC-V environments, so it is discourage
+to use in a portable code. 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.
+
+    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.