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 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 COMPAT_HWCAP_ISA_V bit of 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:

    • 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).

    • PR_RISCV_V_VSTATE_CTRL_ON: Allow Vector to be run for the thread.

    • 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].

    • 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.

    • 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.

    • 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 COMPAT_HWCAP_ISA_V bit of 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