main
1// Copyright 2024 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package cpu
6
7import (
8 "syscall"
9 "unsafe"
10)
11
12// RISC-V extension discovery code for Linux. The approach here is to first try the riscv_hwprobe
13// syscall falling back to HWCAP to check for the C extension if riscv_hwprobe is not available.
14//
15// A note on detection of the Vector extension using HWCAP.
16//
17// Support for the Vector extension version 1.0 was added to the Linux kernel in release 6.5.
18// Support for the riscv_hwprobe syscall was added in 6.4. It follows that if the riscv_hwprobe
19// syscall is not available then neither is the Vector extension (which needs kernel support).
20// The riscv_hwprobe syscall should then be all we need to detect the Vector extension.
21// However, some RISC-V board manufacturers ship boards with an older kernel on top of which
22// they have back-ported various versions of the Vector extension patches but not the riscv_hwprobe
23// patches. These kernels advertise support for the Vector extension using HWCAP. Falling
24// back to HWCAP to detect the Vector extension, if riscv_hwprobe is not available, or simply not
25// bothering with riscv_hwprobe at all and just using HWCAP may then seem like an attractive option.
26//
27// Unfortunately, simply checking the 'V' bit in AT_HWCAP will not work as this bit is used by
28// RISC-V board and cloud instance providers to mean different things. The Lichee Pi 4A board
29// and the Scaleway RV1 cloud instances use the 'V' bit to advertise their support for the unratified
30// 0.7.1 version of the Vector Specification. The Banana Pi BPI-F3 and the CanMV-K230 board use
31// it to advertise support for 1.0 of the Vector extension. Versions 0.7.1 and 1.0 of the Vector
32// extension are binary incompatible. HWCAP can then not be used in isolation to populate the
33// HasV field as this field indicates that the underlying CPU is compatible with RVV 1.0.
34//
35// There is a way at runtime to distinguish between versions 0.7.1 and 1.0 of the Vector
36// specification by issuing a RVV 1.0 vsetvli instruction and checking the vill bit of the vtype
37// register. This check would allow us to safely detect version 1.0 of the Vector extension
38// with HWCAP, if riscv_hwprobe were not available. However, the check cannot
39// be added until the assembler supports the Vector instructions.
40//
41// Note the riscv_hwprobe syscall does not suffer from these ambiguities by design as all of the
42// extensions it advertises support for are explicitly versioned. It's also worth noting that
43// the riscv_hwprobe syscall is the only way to detect multi-letter RISC-V extensions, e.g., Zba.
44// These cannot be detected using HWCAP and so riscv_hwprobe must be used to detect the majority
45// of RISC-V extensions.
46//
47// Please see https://docs.kernel.org/arch/riscv/hwprobe.html for more information.
48
49// golang.org/x/sys/cpu is not allowed to depend on golang.org/x/sys/unix so we must
50// reproduce the constants, types and functions needed to make the riscv_hwprobe syscall
51// here.
52
53const (
54 // Copied from golang.org/x/sys/unix/ztypes_linux_riscv64.go.
55 riscv_HWPROBE_KEY_IMA_EXT_0 = 0x4
56 riscv_HWPROBE_IMA_C = 0x2
57 riscv_HWPROBE_IMA_V = 0x4
58 riscv_HWPROBE_EXT_ZBA = 0x8
59 riscv_HWPROBE_EXT_ZBB = 0x10
60 riscv_HWPROBE_EXT_ZBS = 0x20
61 riscv_HWPROBE_KEY_CPUPERF_0 = 0x5
62 riscv_HWPROBE_MISALIGNED_FAST = 0x3
63 riscv_HWPROBE_MISALIGNED_MASK = 0x7
64)
65
66const (
67 // sys_RISCV_HWPROBE is copied from golang.org/x/sys/unix/zsysnum_linux_riscv64.go.
68 sys_RISCV_HWPROBE = 258
69)
70
71// riscvHWProbePairs is copied from golang.org/x/sys/unix/ztypes_linux_riscv64.go.
72type riscvHWProbePairs struct {
73 key int64
74 value uint64
75}
76
77const (
78 // CPU features
79 hwcap_RISCV_ISA_C = 1 << ('C' - 'A')
80)
81
82func doinit() {
83 // A slice of key/value pair structures is passed to the RISCVHWProbe syscall. The key
84 // field should be initialised with one of the key constants defined above, e.g.,
85 // RISCV_HWPROBE_KEY_IMA_EXT_0. The syscall will set the value field to the appropriate value.
86 // If the kernel does not recognise a key it will set the key field to -1 and the value field to 0.
87
88 pairs := []riscvHWProbePairs{
89 {riscv_HWPROBE_KEY_IMA_EXT_0, 0},
90 {riscv_HWPROBE_KEY_CPUPERF_0, 0},
91 }
92
93 // This call only indicates that extensions are supported if they are implemented on all cores.
94 if riscvHWProbe(pairs, 0) {
95 if pairs[0].key != -1 {
96 v := uint(pairs[0].value)
97 RISCV64.HasC = isSet(v, riscv_HWPROBE_IMA_C)
98 RISCV64.HasV = isSet(v, riscv_HWPROBE_IMA_V)
99 RISCV64.HasZba = isSet(v, riscv_HWPROBE_EXT_ZBA)
100 RISCV64.HasZbb = isSet(v, riscv_HWPROBE_EXT_ZBB)
101 RISCV64.HasZbs = isSet(v, riscv_HWPROBE_EXT_ZBS)
102 }
103 if pairs[1].key != -1 {
104 v := pairs[1].value & riscv_HWPROBE_MISALIGNED_MASK
105 RISCV64.HasFastMisaligned = v == riscv_HWPROBE_MISALIGNED_FAST
106 }
107 }
108
109 // Let's double check with HWCAP if the C extension does not appear to be supported.
110 // This may happen if we're running on a kernel older than 6.4.
111
112 if !RISCV64.HasC {
113 RISCV64.HasC = isSet(hwCap, hwcap_RISCV_ISA_C)
114 }
115}
116
117func isSet(hwc uint, value uint) bool {
118 return hwc&value != 0
119}
120
121// riscvHWProbe is a simplified version of the generated wrapper function found in
122// golang.org/x/sys/unix/zsyscall_linux_riscv64.go. We simplify it by removing the
123// cpuCount and cpus parameters which we do not need. We always want to pass 0 for
124// these parameters here so the kernel only reports the extensions that are present
125// on all cores.
126func riscvHWProbe(pairs []riscvHWProbePairs, flags uint) bool {
127 var _zero uintptr
128 var p0 unsafe.Pointer
129 if len(pairs) > 0 {
130 p0 = unsafe.Pointer(&pairs[0])
131 } else {
132 p0 = unsafe.Pointer(&_zero)
133 }
134
135 _, _, e1 := syscall.Syscall6(sys_RISCV_HWPROBE, uintptr(p0), uintptr(len(pairs)), uintptr(0), uintptr(0), uintptr(flags), 0)
136 return e1 == 0
137}