Update

vendor/tailscale.com/tsweb/varz/goamd64_linux_amd64.go

@@ -0,0 +1,88 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !android
+
+package varz
+
+import (
+	"bytes"
+	"expvar"
+	"os"
+	"runtime/debug"
+	"strings"
+)
+
+func init() {
+	var capable any = hostGOAMD64Level()
+	expvar.Publish("gauge_goamd64_capable", expvar.Func(func() any { return capable }))
+	var compiled any = compiledGOAMD64Level()
+	expvar.Publish("gauge_goamd64_compiled", expvar.Func(func() any { return compiled }))
+}
+
+// goamd64LevelFlags maps each x86-64 microarchitecture level to the
+// /proc/cpuinfo flag names it requires beyond the previous level.
+// It matches the per-level feature checks in Go's runtime/asm_amd64.s.
+// Note that /proc/cpuinfo spells SSE3 as "pni" and LZCNT as "abm",
+// and the kernel clears the AVX and AVX-512 flags when the OS lacks
+// XSAVE support, so OSXSAVE needs no separate check.
+var goamd64LevelFlags = [...][]string{
+	2: {"cx16", "lahf_lm", "popcnt", "pni", "sse4_1", "sse4_2", "ssse3"},
+	3: {"abm", "avx", "avx2", "bmi1", "bmi2", "f16c", "fma", "movbe"},
+	4: {"avx512f", "avx512bw", "avx512cd", "avx512dq", "avx512vl"},
+}
+
+// hostGOAMD64Level returns the maximum GOAMD64 microarchitecture
+// level (1-4) that the host CPU supports, regardless of what level
+// this binary was compiled for. It returns 0 on error.
+func hostGOAMD64Level() int {
+	cpuinfo, err := os.ReadFile("/proc/cpuinfo")
+	if err != nil {
+		return 0
+	}
+	for line := range bytes.Lines(cpuinfo) {
+		name, rest, ok := bytes.Cut(line, []byte(":"))
+		if ok && string(bytes.TrimSpace(name)) == "flags" {
+			return goamd64Level(strings.Fields(string(rest)))
+		}
+	}
+	return 0
+}
+
+// goamd64Level returns the maximum GOAMD64 level (1-4) supported by
+// a CPU with the given /proc/cpuinfo flags.
+func goamd64Level(flags []string) int {
+	has := make(map[string]bool, len(flags))
+	for _, f := range flags {
+		has[f] = true
+	}
+	level := 1
+	for next := 2; next < len(goamd64LevelFlags); next++ {
+		for _, f := range goamd64LevelFlags[next] {
+			if !has[f] {
+				return level
+			}
+		}
+		level = next
+	}
+	return level
+}
+
+// compiledGOAMD64Level returns the GOAMD64 microarchitecture level
+// (1-4) that this binary was compiled for, as recorded in its build
+// info. It returns 0 if unknown.
+func compiledGOAMD64Level() int {
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		return 0
+	}
+	for _, s := range bi.Settings {
+		if s.Key == "GOAMD64" {
+			if v, ok := strings.CutPrefix(s.Value, "v"); ok && len(v) == 1 && v[0] >= '1' && v[0] <= '4' {
+				return int(v[0] - '0')
+			}
+			return 0
+		}
+	}
+	return 0
+}