feat(device): implement MSR fallback for CPU power meter

This commit implements EP-002 MSR fallback power meter enhancement proposal.
Add MSR (Model Specific Register) support as fallback when powercap interface
is unavailable. This enhancement improves Kepler's compatibility across
different systems and kernel configurations.

Key changes:
- Add MSR reader implementation with Intel RAPL register support
- Create raplReader interface abstracting powercap and MSR backends
- Extract existing powercap logic into dedicated reader component
- Enhance RAPL power meter with automatic fallback detection
- Add MSR configuration with security-conscious opt-in defaults
- Implement comprehensive test coverage with mock MSR data

The MSR fallback is disabled by default due to PLATYPUS attack vectors
(CVE-2020-8694/8695) and must be explicitly enabled via configuration.
When enabled, the system automatically falls back to MSR if powercap
is unavailable, maintaining transparent operation.

Signed-off-by: Sunil Thaha <[email protected]>

Author: sthaha

cmd/kepler/main.go

@@ -233,9 +233,17 @@ func createCPUMeter(logger *slog.Logger, cfg *config.Config) (device.CPUPowerMet
 		logger.Info("rapl zones are filtered", "zones-enabled", cfg.Rapl.Zones)
 	}
 
+	// Convert config MSR settings to device MSRConfig
+	msrConfig := device.MSRConfig{
+		Enabled:    cfg.MSR.Enabled,
+		Force:      cfg.MSR.Force,
+		DevicePath: cfg.MSR.DevicePath,
+	}
+
 	return device.NewCPUPowerMeter(
 		cfg.Host.SysFS,
 		device.WithRaplLogger(logger),
 		device.WithZoneFilter(cfg.Rapl.Zones),
+		device.WithMSRConfig(msrConfig),
 	)
 }

config/config.go

@@ -33,6 +33,16 @@ type (
 		Zones []string `yaml:"zones"`
 	}
 
+	// MSR configuration for fallback power reading
+	MSR struct {
+		// Enable automatic MSR fallback when powercap unavailable
+		Enabled *bool `yaml:"enabled"`
+		// Force MSR usage even if powercap available (testing)
+		Force *bool `yaml:"force"`
+		// MSR device path template
+		DevicePath string `yaml:"devicePath"`
+	}
+
 	// Development mode settings; disabled by default
 	Dev struct {
 		FakeCpuMeter struct {
@@ -98,6 +108,7 @@ type (
 		Host     Host     `yaml:"host"`
 		Monitor  Monitor  `yaml:"monitor"`
 		Rapl     Rapl     `yaml:"rapl"`
+		MSR      MSR      `yaml:"msr"`
 		Exporter Exporter `yaml:"exporter"`
 		Web      Web      `yaml:"web"`
 		Debug    Debug    `yaml:"debug"`
@@ -168,6 +179,12 @@ const (
 	// RAPL
 	RaplZones = "rapl.zones" // not a flag
 
+	// MSR - NOTE: MSR settings are not exposed as CLI flags per proposal
+	// They should only be configured via YAML files due to security implications
+	MSREnabled    = "msr.enabled"    // not a flag
+	MSRForce      = "msr.force"      // not a flag
+	MSRDevicePath = "msr.devicePath" // not a flag
+
 	pprofEnabledFlag = "debug.pprof"
 
 	WebConfigFlag        = "web.config-file"
@@ -203,6 +220,11 @@ func DefaultConfig() *Config {
 		Rapl: Rapl{
 			Zones: []string{},
 		},
+		MSR: MSR{
+			Enabled:    ptr.To(false), // Opt-in for security
+			Force:      ptr.To(false),
+			DevicePath: "/dev/cpu/%d/msr",
+		},
 		Monitor: Monitor{
 			Interval:  5 * time.Second,
 			Staleness: 500 * time.Millisecond,
@@ -408,6 +430,9 @@ func (c *Config) sanitize() {
 		c.Rapl.Zones[i] = strings.TrimSpace(c.Rapl.Zones[i])
 	}
 
+	// MSR settings sanitization
+	c.MSR.DevicePath = strings.TrimSpace(c.MSR.DevicePath)
+
 	for i := range c.Exporter.Prometheus.DebugCollectors {
 		c.Exporter.Prometheus.DebugCollectors[i] = strings.TrimSpace(c.Exporter.Prometheus.DebugCollectors[i])
 	}
@@ -488,6 +513,16 @@ func (c *Config) Validate(skips ...SkipValidation) error {
 			errs = append(errs, fmt.Sprintf("invalid monitor min terminated energy threshold: %d can't be negative", c.Monitor.MinTerminatedEnergyThreshold))
 		}
 	}
+	{ // MSR settings
+		if c.MSR.DevicePath == "" {
+			errs = append(errs, "MSR device path cannot be empty")
+		} else {
+			// Basic validation that device path is a template
+			if !strings.Contains(c.MSR.DevicePath, "%d") {
+				errs = append(errs, "MSR device path must contain '%d' placeholder for CPU ID")
+			}
+		}
+	}
 	{ // Kubernetes
 		if ptr.Deref(c.Kube.Enabled, false) {
 			if c.Kube.Config != "" {

internal/device/mock_cpu_power_meter.go

@@ -5,14 +5,6 @@ package device
 
 // TODO: Move this mock to a separate testutil package
 
-import (
-	"slices"
-	"testing"
-
-	"github.com/prometheus/procfs/sysfs"
-	"github.com/stretchr/testify/require"
-)
-
 const (
 	validSysFSPath = "testdata/sys"
 	badSysFSPath   = "testdata/bad_sysfs"
@@ -67,27 +59,3 @@ func (m *MockRaplZone) OnEnergy(j Energy, err error) {
 func (m *MockRaplZone) Inc(delta Energy) {
 	m.energy = (m.energy + delta) % m.maxMicroJoules
 }
-
-func validSysFSFixtures(t *testing.T) sysfs.FS {
-	t.Helper()
-	fs, err := sysfs.NewFS(validSysFSPath)
-	require.NoError(t, err, "Failed to create sysfs test FS")
-	return fs
-}
-
-func invalidSysFSFixtures(t *testing.T) sysfs.FS {
-	t.Helper()
-	fs, err := sysfs.NewFS(badSysFSPath)
-	require.NoError(t, err, "Failed to create sysfs test FS")
-	return fs
-}
-
-func sortedZoneNames(zones []EnergyZone) []string {
-	names := make([]string, len(zones))
-	for i, zone := range zones {
-		names[i] = zone.Name()
-	}
-	slices.Sort(names)
-
-	return names
-}