virtcontainers: Implement function to get the pmem DeviceInfo

Implement function to get the pmem `DeviceInfo` from a volume.
`PmemDeviceInfo` return a new `DeviceInfo` object if a volume has a loop device
as backend and the backing file for such loop device contains the PFN signature,
needed to enable DAX in the guest.

Signed-off-by: Julio Montes <[email protected]>

virtcontainers/device/config/config.go

@@ -113,6 +113,10 @@ type DeviceInfo struct {
 	Major int64
 	Minor int64
 
+	// Pmem enabled persistent memory. Use HostPath as backing file
+	// for a nvdimm device in the guest.
+	Pmem bool
+
 	// FileMode permission bits for the device.
 	FileMode os.FileMode
 
@@ -169,6 +173,10 @@ type BlockDrive struct {
 
 	// ReadOnly sets the device file readonly
 	ReadOnly bool
+
+	// Pmem enables persistent memory. Use File as backing file
+	// for a nvdimm device in the guest
+	Pmem bool
 }
 
 // VFIODeviceType indicates VFIO device type

virtcontainers/device/config/pmem.go

@@ -0,0 +1,116 @@
+// Copyright (c) 2020 Intel Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package config
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	"github.com/kata-containers/runtime/virtcontainers/utils"
+	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
+)
+
+const (
+	// This signature is defined in the linux NVDIMM driver.
+	// devices or backing files with this signature can be used
+	// as pmem (persistent memory) devices in the guest.
+	pfnSignature = "NVDIMM_PFN_INFO"
+
+	// offset in the backing file where the signature should be
+	pfnSignatureOffset = int64(4 * 1024)
+)
+
+var (
+	pmemLog = logrus.WithField("source", "virtcontainers/device/config")
+)
+
+// PmemDeviceInfo returns a DeviceInfo if a loop device
+// is mounted on source, and the backing file of the loop device
+// has the PFN signature.
+func PmemDeviceInfo(source, destination string) (*DeviceInfo, error) {
+	stat := syscall.Stat_t{}
+	err := syscall.Stat(source, &stat)
+	if err != nil {
+		return nil, err
+	}
+
+	// device object is still incomplete,
+	// but it can be used to fetch the backing file
+	device := &DeviceInfo{
+		ContainerPath: destination,
+		DevType:       "b",
+		Major:         int64(unix.Major(stat.Dev)),
+		Minor:         int64(unix.Minor(stat.Dev)),
+		Pmem:          true,
+		DriverOptions: make(map[string]string),
+	}
+
+	pmemLog.WithFields(
+		logrus.Fields{
+			"major": device.Major,
+			"minor": device.Minor,
+		}).Debug("looking for backing file")
+
+	device.HostPath, err = getBackingFile(*device)
+	if err != nil {
+		return nil, err
+	}
+
+	pmemLog.WithField("backing-file", device.HostPath).
+		Debug("backing file found: looking for PFN signature")
+
+	if !hasPFNSignature(device.HostPath) {
+		return nil, fmt.Errorf("backing file %v has not PFN signature", device.HostPath)
+	}
+
+	_, fstype, err := utils.GetDevicePathAndFsType(source)
+	if err != nil {
+		pmemLog.WithError(err).WithField("mount-point", source).Warn("failed to get fstype: using ext4")
+		fstype = "ext4"
+	}
+
+	pmemLog.WithField("fstype", fstype).Debug("filesystem for mount point")
+	device.DriverOptions["fstype"] = fstype
+
+	return device, nil
+}
+
+// returns true if the file/device path has the PFN signature
+// required to use it as PMEM device and enable DAX.
+// See [1] to know more about the PFN signature.
+//
+// [1] - https://github.com/kata-containers/osbuilder/blob/master/image-builder/nsdax.gpl.c
+func hasPFNSignature(path string) bool {
+	f, err := os.Open(path)
+	if err != nil {
+		pmemLog.WithError(err).Error("Could not get PFN signature")
+		return false
+	}
+	defer f.Close()
+
+	signatureLen := len(pfnSignature)
+	signature := make([]byte, signatureLen)
+
+	l, err := f.ReadAt(signature, pfnSignatureOffset)
+	if err != nil {
+		pmemLog.WithError(err).Debug("Could not read pfn signature")
+		return false
+	}
+
+	pmemLog.WithFields(logrus.Fields{
+		"path":      path,
+		"signature": string(signature),
+	}).Debug("got signature")
+
+	if l != signatureLen {
+		pmemLog.WithField("read-bytes", l).Debug("Incomplete signature")
+		return false
+	}
+
+	return pfnSignature == string(signature)
+}

virtcontainers/device/config/pmem_test.go

@@ -0,0 +1,49 @@
+// Copyright (c) 2020 Intel Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package config
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func createPFNFile(assert *assert.Assertions, dir string) string {
+	pfnPath := filepath.Join(dir, "pfn")
+	file, err := os.Create(pfnPath)
+	assert.NoError(err)
+	defer file.Close()
+
+	l, err := file.WriteAt([]byte(pfnSignature), pfnSignatureOffset)
+	assert.NoError(err)
+	assert.Equal(len(pfnSignature), l)
+
+	return pfnPath
+}
+
+func TestHasPFNSignature(t *testing.T) {
+	assert := assert.New(t)
+
+	b := hasPFNSignature("/abc/xyz/123/sw")
+	assert.False(b)
+
+	f, err := ioutil.TempFile("", "pfn")
+	assert.NoError(err)
+	f.Close()
+	defer os.Remove(f.Name())
+
+	b = hasPFNSignature(f.Name())
+	assert.False(b)
+
+	pfnFile := createPFNFile(assert, os.TempDir())
+	defer os.Remove(pfnFile)
+
+	b = hasPFNSignature(pfnFile)
+	assert.True(b)
+}