// Copyright 2024 The Go Authors. All rights reserved.

// Use of this source code is governed by a BSD-style

// license that can be found in the LICENSE file.

//go:build riscv64 && linux

package cpu

import _ "unsafe"

// RISC-V extension discovery code for Linux.

//

// A note on detection of the Vector extension using HWCAP.

//

// Support for the Vector extension version 1.0 was added to the Linux kernel in release 6.5.

// Support for the riscv_hwprobe syscall was added in 6.4. It follows that if the riscv_hwprobe

// syscall is not available then neither is the Vector extension (which needs kernel support).

// The riscv_hwprobe syscall should then be all we need to detect the Vector extension.

// However, some RISC-V board manufacturers ship boards with an older kernel on top of which

// they have back-ported various versions of the Vector extension patches but not the riscv_hwprobe

// patches. These kernels advertise support for the Vector extension using HWCAP. Falling

// back to HWCAP to detect the Vector extension, if riscv_hwprobe is not available, or simply not

// bothering with riscv_hwprobe at all and just using HWCAP may then seem like an attractive option.

//

// Unfortunately, simply checking the 'V' bit in AT_HWCAP will not work as this bit is used by

// RISC-V board and cloud instance providers to mean different things. The Lichee Pi 4A board

// and the Scaleway RV1 cloud instances use the 'V' bit to advertise their support for the unratified

// 0.7.1 version of the Vector Specification. The Banana Pi BPI-F3 and the CanMV-K230 board use

// it to advertise support for 1.0 of the Vector extension. Versions 0.7.1 and 1.0 of the Vector

// extension are binary incompatible. HWCAP can then not be used in isolation to populate the

// HasV field as this field indicates that the underlying CPU is compatible with RVV 1.0.

// Go will only support the ratified versions >= 1.0 and so any vector code it might generate

// would crash on a Scaleway RV1 instance or a Lichee Pi 4a, if allowed to run.

//

// There is a way at runtime to distinguish between versions 0.7.1 and 1.0 of the Vector

// specification by issuing a RVV 1.0 vsetvli instruction and checking the vill bit of the vtype

// register. This check would allow us to safely detect version 1.0 of the Vector extension

// with HWCAP, if riscv_hwprobe were not available. However, the check cannot

// be added until the assembler supports the Vector instructions.

//

// Note the riscv_hwprobe syscall does not suffer from these ambiguities by design as all of the

// extensions it advertises support for are explicitly versioned. It's also worth noting that

// the riscv_hwprobe syscall is the only way to detect multi-letter RISC-V extensions, e.g., Zvbb.

// These cannot be detected using HWCAP and so riscv_hwprobe must be used to detect the majority

// of RISC-V extensions.

//

// Please see https://docs.kernel.org/arch/riscv/hwprobe.html for more information.

const (

// Copied from golang.org/x/sys/unix/ztypes_linux_riscv64.go.

riscv_HWPROBE_KEY_IMA_EXT_0 = 0x4

riscv_HWPROBE_IMA_V = 0x4

riscv_HWPROBE_KEY_CPUPERF_0 = 0x5

riscv_HWPROBE_MISALIGNED_FAST = 0x3

riscv_HWPROBE_MISALIGNED_MASK = 0x7

)

// riscvHWProbePairs is copied from golang.org/x/sys/unix/ztypes_linux_riscv64.go.

type riscvHWProbePairs struct {

key int64

value uint64

}

//go:linkname riscvHWProbe

func riscvHWProbe(pairs []riscvHWProbePairs, flags uint) bool

func osInit() {

}