Documentation/arch/riscv/vector.rst - linux - Git at Google

 .. 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 these interfaces is not
 recommended in libraries routines because libraries should not override policies
 configured from the parent process. Also, users must note 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.

 3.  Vector Register State Across System Calls
 ---------------------------------------------

 As indicated by version 1.0 of the V extension [1], vector registers are
 clobbered by system calls.

 1: https://github.com/riscv/riscv-v-spec/blob/master/calling-convention.adoc
	.. 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 these interfaces is not
	recommended in libraries routines because libraries should not override policies
	configured from the parent process. Also, users must note 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.

	3. Vector Register State Across System Calls
	---------------------------------------------

	As indicated by version 1.0 of the V extension [1], vector registers are
	clobbered by system calls.

	1: https://github.com/riscv/riscv-v-spec/blob/master/calling-convention.adoc