First implementation

vendor/periph.io/x/periph/host/pmem/alloc.go

@@ -0,0 +1,182 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package pmem
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"reflect"
+	"sync"
+	"unsafe"
+)
+
+const pageSize = 4096
+
+// Mem represents a section of memory that is usable by the DMA controller.
+//
+// Since this is physically allocated memory, that could potentially have been
+// allocated in spite of OS consent, for example by asking the GPU directly, it
+// is important to call Close() before process exit.
+type Mem interface {
+	io.Closer
+	// Bytes returns the user space memory mapped buffer address as a slice of
+	// bytes.
+	//
+	// It is the raw view of the memory from this process.
+	Bytes() []byte
+	// AsPOD initializes a pointer to a POD (plain old data) to point to the
+	// memory mapped region.
+	//
+	// pp must be a pointer to:
+	//
+	// - pointer to a base size type (uint8, int64, float32, etc)
+	// - struct
+	// - array of the above
+	// - slice of the above
+	//
+	// and the value must be nil. Returns an error otherwise.
+	//
+	// If a pointer to a slice is passed in, it is initialized to the length and
+	// capacity set to the maximum number of elements this slice can represent.
+	//
+	// The pointer initialized points to the same address as Bytes().
+	AsPOD(pp interface{}) error
+	// PhysAddr is the physical address. It can be either 32 bits or 64 bits,
+	// depending on the bitness of the OS kernel, not on the user mode build,
+	// e.g. you could have compiled on a 32 bits Go toolchain but running on a
+	// 64 bits kernel.
+	PhysAddr() uint64
+}
+
+// MemAlloc represents contiguous physically locked memory that was allocated.
+//
+// The memory is mapped in user space.
+//
+// MemAlloc implements Mem.
+type MemAlloc struct {
+	View
+}
+
+// Close unmaps the physical memory allocation.
+func (m *MemAlloc) Close() error {
+	if err := munlock(m.orig); err != nil {
+		return err
+	}
+	return munmap(m.orig)
+}
+
+// Alloc allocates a continuous chunk of physical memory.
+//
+// Size must be rounded to 4Kb. Allocations of 4Kb will normally succeed.
+// Allocations larger than 64Kb will likely fail due to kernel memory
+// fragmentation; rebooting the host or reducing the number of running programs
+// may help.
+//
+// The allocated memory is uncached.
+func Alloc(size int) (*MemAlloc, error) {
+	if size == 0 || size&(pageSize-1) != 0 {
+		return nil, wrapf("allocated memory must be rounded to %d bytes", pageSize)
+	}
+	if isLinux && !isWSL() {
+		return allocLinux(size)
+	}
+	return nil, wrapf("memory allocation is not supported on this platform")
+}
+
+//
+
+var (
+	wslOnce    sync.Once
+	isWSLValue bool
+)
+
+// uallocMemLocked allocates user space memory and requests the OS to have the
+// chunk to be locked into physical memory.
+func uallocMemLocked(size int) ([]byte, error) {
+	// It is important to write to the memory so it is forced to be present.
+	b, err := uallocMem(size)
+	if err == nil {
+		for i := range b {
+			b[i] = 0
+		}
+		if err := mlock(b); err != nil {
+			// Ignore the unmap error.
+			_ = munmap(b)
+			return nil, wrapf("locking %d bytes failed: %v", size, err)
+		}
+	}
+	return b, err
+}
+
+// allocLinux allocates physical memory and returns a user view to it.
+func allocLinux(size int) (*MemAlloc, error) {
+	// TODO(maruel): Implement the "shotgun approach". Allocate a ton of 4Kb
+	// pages and lock them. Then look at their physical pages and only keep the
+	// one useful. Then create a linear mapping in memory to simplify the user
+	// mode with a single linear user space virtual address but keep the
+	// individual page alive with their initial allocation. When done release
+	// each individual page.
+	if size > pageSize {
+		return nil, wrapf("large allocation is not yet implemented")
+	}
+	// First allocate a chunk of user space memory.
+	b, err := uallocMemLocked(size)
+	if err != nil {
+		return nil, err
+	}
+	pages := make([]uint64, (size+pageSize-1)/pageSize)
+	// Figure out the physical memory addresses.
+	for i := range pages {
+		pages[i], err = virtToPhys(toRaw(b[pageSize*i:]))
+		if err != nil {
+			return nil, err
+		}
+		if pages[i] == 0 {
+			return nil, wrapf("failed to read page %d", i)
+		}
+	}
+	for i := 1; i < len(pages); i++ {
+		// Fail if the memory is not contiguous.
+		if pages[i] != pages[i-1]+pageSize {
+			return nil, wrapf("failed to allocate %d bytes of continugous physical memory; page %d =0x%x; page %d=0x%x", size, i, pages[i], i-1, pages[i-1])
+		}
+	}
+
+	return &MemAlloc{View{Slice: b, phys: pages[0], orig: b}}, nil
+}
+
+// virtToPhys returns the physical memory address backing a virtual
+// memory address.
+func virtToPhys(virt uintptr) (uint64, error) {
+	physPage, err := ReadPageMap(virt)
+	if err != nil {
+		return 0, err
+	}
+	if physPage&(1<<63) == 0 {
+		// If high bit is not set, the page doesn't exist.
+		return 0, wrapf("0x%08x has no physical address", virt)
+	}
+	// Strip flags. See linux documentation on kernel.org for more details.
+	physPage &^= 0x1FF << 55
+	return physPage * pageSize, nil
+}
+
+func toRaw(b []byte) uintptr {
+	header := *(*reflect.SliceHeader)(unsafe.Pointer(&b))
+	return header.Data
+}
+
+// isWSL returns true if running under Windows Subsystem for Linux.
+func isWSL() bool {
+	wslOnce.Do(func() {
+		if c, err := ioutil.ReadFile("/proc/sys/kernel/osrelease"); err == nil {
+			isWSLValue = bytes.Contains(c, []byte("Microsoft"))
+		}
+	})
+	return isWSLValue
+}
+
+var _ Mem = &MemAlloc{}

vendor/periph.io/x/periph/host/pmem/doc.go

@@ -0,0 +1,69 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// Package pmem implements handling of physical memory for user space programs.
+//
+// To make things confusing, a modern computer has many view of the memory
+// (address spaces):
+//
+// User
+//
+// User mode address space is the virtual address space that an application
+// runs in.  It is generally a tad less than half the addressable memory, so on
+// a 32 bits system, the addressable range is 1.9Gb. For 64 bits OS, it depends
+// but it usually at least 3.5Gb. The memory is virtual and can be flushed to
+// disk in the swap file unless individual pages are locked.
+//
+// Kernel
+//
+// Kernel address space is the virtual address space the kernel sees. It often
+// can see the currently active user space program on the current CPU core in
+// addition to all the memory the kernel sees. The kernel memory pages that are
+// not mlock()'ed are 'virtual' and can be flushed to disk in the swap file
+// when there's not enough RAM available. On linux systems, the kernel
+// addressed memory can be mapped in user space via `/dev/kmem`.
+//
+// Physical
+//
+// Physical memory address space is the actual address of each page in the DRAM
+// chip and anything connected to the memory controller. The mapping may be
+// different depending on what controller looks at the bus, like with IOMMU. So
+// a peripheral (GPU, DMA controller) may have a different view of the physical
+// memory than the host CPU. On linux systems, this memory can be mapped in
+// user space via `/dev/mem`.
+//
+// CPU
+//
+// The CPU or its subsystems may memory map registers (for example, to control
+// GPIO pins, clock speed, etc). This is not "real" memory, this is a view of
+// registers but it still follows "mostly" the same semantic as DRAM backed
+// physical memory.
+//
+// Some CPU memory may have very special semantic where the mere fact of
+// reading has side effects. For example reading a specific register may
+// latches another.
+//
+// CPU memory accesses are layered with multiple caches, usually named L1, L2
+// and optionally L3. Some controllers (DMA) can see some cache levels (L2) but
+// not others (L1) on some CPU architecture (bcm283x). This means that a user
+// space program writing data to a memory page and immediately asking the DMA
+// controller to read it may cause stale data to be read!
+//
+// Hypervisor
+//
+// Hypervisor can change the complete memory mapping as seen by the kernel.
+// This is outside the scope of this project. :)
+//
+// Summary
+//
+// In practice, the semantics change between CPU manufacturers (Broadcom vs
+// Allwinner) and between architectures (ARM vs x86). The most tricky one is to
+// understand cached memory and how it affects coherence and performance.
+// Uncached memory is extremely slow so it must only be used when necessary.
+//
+// References
+//
+// Overview of IOMMU:
+// https://en.wikipedia.org/wiki/Input-output_memory_management_unit
+package pmem

vendor/periph.io/x/periph/host/pmem/mem_linux.go

@@ -0,0 +1,56 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package pmem
+
+import "syscall"
+
+const isLinux = true
+
+func mmap(fd uintptr, offset int64, length int) ([]byte, error) {
+	v, err := syscall.Mmap(int(fd), offset, length, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
+	if err != nil {
+		return nil, wrapf("failed to memory map: %v", err)
+	}
+	return v, nil
+}
+
+func munmap(b []byte) error {
+	if err := syscall.Munmap(b); err != nil {
+		return wrapf("failed to unmap memory: %v", err)
+	}
+	return nil
+
+}
+
+func mlock(b []byte) error {
+	if err := syscall.Mlock(b); err != nil {
+		return wrapf("failed to lock memory: %v", err)
+	}
+	return nil
+}
+
+func munlock(b []byte) error {
+	if err := syscall.Munlock(b); err != nil {
+		return wrapf("failed to unlock memory: %v", err)
+	}
+	return nil
+}
+
+// uallocMem allocates user space memory.
+func uallocMem(size int) ([]byte, error) {
+	b, err := syscall.Mmap(
+		0,
+		0,
+		size,
+		syscall.PROT_READ|syscall.PROT_WRITE,
+		syscall.MAP_ANONYMOUS|syscall.MAP_LOCKED|syscall.MAP_NORESERVE|syscall.MAP_SHARED)
+	// syscall.MAP_HUGETLB / MAP_HUGE_2MB
+	// See /sys/kernel/mm/hugepages but both C.H.I.P. running Jessie and Raspbian
+	// Jessie do not expose huge pages. :(
+	if err != nil {
+		return nil, wrapf("allocating %d bytes failed: %v", size, err)
+	}
+	return b, err
+}

vendor/periph.io/x/periph/host/pmem/mem_other.go

@@ -0,0 +1,30 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// +build !linux
+
+package pmem
+
+const isLinux = false
+
+func mmap(fd uintptr, offset int64, length int) ([]byte, error) {
+	return nil, wrapf("syscall.Mmap() not implemented on this OS")
+}
+
+func munmap(b []byte) error {
+	return wrapf("syscall.Munmap() not implemented on this OS")
+}
+
+func mlock(b []byte) error {
+	return wrapf("syscall.Mlock() not implemented on this OS")
+}
+
+func munlock(b []byte) error {
+	return wrapf("syscall.Munlock() not implemented on this OS")
+}
+
+// uallocMem allocates user space memory.
+func uallocMem(size int) ([]byte, error) {
+	return make([]byte, size), nil
+}

vendor/periph.io/x/periph/host/pmem/pagemap.go

@@ -0,0 +1,64 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package pmem
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"os"
+)
+
+// ReadPageMap reads a physical address mapping for a virtual page address from
+// /proc/self/pagemap.
+//
+// It returns the physical address that corresponds to the start of the virtual
+// page within which the virtual address virtAddr is located.
+//
+// The meaning of the return value is documented at
+// https://www.kernel.org/doc/Documentation/vm/pagemap.txt
+func ReadPageMap(virtAddr uintptr) (uint64, error) {
+	if !isLinux || isWSL() {
+		return 0, errors.New("pmem: pagemap is not supported on this platform")
+	}
+	return readPageMapLinux(virtAddr)
+}
+
+//
+
+var (
+	pageMap    fileIO
+	pageMapErr error
+)
+
+func readPageMapLinux(virtAddr uintptr) (uint64, error) {
+	var b [8]byte
+	mu.Lock()
+	defer mu.Unlock()
+	if pageMap == nil && pageMapErr == nil {
+		// Open /proc/self/pagemap.
+		//
+		// It is a uint64 array where the index represents the virtual 4Kb page
+		// number and the value represents the physical page properties backing
+		// this virtual page.
+		pageMap, pageMapErr = openFile("/proc/self/pagemap", os.O_RDONLY|os.O_SYNC)
+	}
+	if pageMapErr != nil {
+		return 0, pageMapErr
+	}
+	// Convert address to page number, then index in uint64 array.
+	offset := int64(virtAddr / pageSize * 8)
+	if _, err := pageMap.Seek(offset, os.SEEK_SET); err != nil {
+		return 0, fmt.Errorf("pmem: failed to seek at 0x%x for 0x%x: %v", offset, virtAddr, err)
+	}
+	n, err := pageMap.Read(b[:])
+	if err != nil {
+		return 0, fmt.Errorf("pmem: failed to read at 0x%x for 0x%x: %v", offset, virtAddr, err)
+	}
+	if n != len(b) {
+		return 0, fmt.Errorf("pmem: failed to read the amount of data %d", len(b))
+	}
+	return binary.LittleEndian.Uint64(b[:]), nil
+}

vendor/periph.io/x/periph/host/pmem/smoketest.go

@@ -0,0 +1,89 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package pmem
+
+import (
+	"bytes"
+	"math/rand"
+)
+
+// TestCopy is used by CPU drivers to verify that the DMA engine works
+// correctly.
+//
+// It is not meant to be used by end users.
+//
+// TestCopy allocates two buffer via `alloc`, once as the source and one as the
+// destination. It fills the source with random data and the destination with
+// 0x11.
+//
+// `copyMem` is expected to copy the memory from pSrc to pDst, with an offset
+// of `hole` and size `size-2*hole`.
+//
+// The function `copyMem` being tested is only given the buffer physical
+// addresses and must copy the data without other help. It is expected to
+//
+// This confirm misaligned DMA copying works.
+// leverage the host's DMA engine.
+func TestCopy(size, holeSize int, alloc func(size int) (Mem, error), copyMem func(pDst, pSrc uint64) error) error {
+	pSrc, err2 := alloc(size)
+	if err2 != nil {
+		return err2
+	}
+	defer pSrc.Close()
+	pDst, err2 := alloc(size)
+	if err2 != nil {
+		return err2
+	}
+	defer pDst.Close()
+	dst := pDst.Bytes()
+	for i := range dst {
+		dst[i] = 0x11
+	}
+	src := make([]byte, size)
+	for i := range src {
+		src[i] = byte(rand.Int31())
+	}
+	copy(pSrc.Bytes(), src[:])
+
+	// Run the driver supplied memory copying code.
+	if err := copyMem(pDst.PhysAddr(), pSrc.PhysAddr()); err != nil {
+		return err
+	}
+
+	// Verifications.
+	for i := 0; i < holeSize; i++ {
+		if dst[i] != 0x11 {
+			return wrapf("DMA corrupted the buffer header: %x", dst[:holeSize])
+		}
+		if dst[size-1-i] != 0x11 {
+			return wrapf("DMA corrupted the buffer footer: %x", dst[size-1-holeSize:])
+		}
+	}
+
+	// Headers and footers were not corupted in the destination. Verify the inner
+	// view that should match.
+	x := src[:size-2*holeSize]
+	y := dst[holeSize : size-holeSize]
+	if !bytes.Equal(x, y) {
+		offset := 0
+		for len(x) != 0 && x[0] == y[0] {
+			x = x[1:]
+			y = y[1:]
+			offset++
+		}
+		for len(x) != 0 && x[len(x)-1] == y[len(y)-1] {
+			x = x[:len(x)-1]
+			y = y[:len(y)-1]
+		}
+		if len(x) > 32 {
+			x = x[:32]
+		}
+		if len(y) > 32 {
+			y = y[:32]
+		}
+		return wrapf("DMA corrupted the buffer at offset %d:\n%x\n%x", offset, x, y)
+	}
+	return nil
+}

vendor/periph.io/x/periph/host/pmem/view.go

@@ -0,0 +1,283 @@
+// Copyright 2016 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package pmem
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"sync"
+	"unsafe"
+
+	"periph.io/x/periph/host/fs"
+)
+
+// Slice can be transparently viewed as []byte, []uint32 or a struct.
+type Slice []byte
+
+// Uint32 returns a view of the byte slice as a []uint32.
+func (s *Slice) Uint32() []uint32 {
+	header := *(*reflect.SliceHeader)(unsafe.Pointer(s))
+	header.Len /= 4
+	header.Cap /= 4
+	return *(*[]uint32)(unsafe.Pointer(&header))
+}
+
+// Bytes implements Mem.
+func (s *Slice) Bytes() []byte {
+	return *s
+}
+
+// AsPOD implements Mem.
+func (s *Slice) AsPOD(pp interface{}) error {
+	if pp == nil {
+		return wrapf("require Ptr, got nil")
+	}
+	vpp := reflect.ValueOf(pp)
+	if elemSize, err := isPS(len(*s), vpp); err == nil {
+		p := vpp.Elem()
+		t := p.Type().Elem()
+		if elemSize > len(*s) {
+			return wrapf("can't map slice of struct %s (size %d) on [%d]byte", t, elemSize, len(*s))
+		}
+		nbElems := len(*s) / elemSize
+		// Use casting black magic to set the internal slice headers.
+		hdr := (*reflect.SliceHeader)(unsafe.Pointer(p.UnsafeAddr()))
+		hdr.Data = ((*reflect.SliceHeader)(unsafe.Pointer(s))).Data
+		hdr.Len = nbElems
+		hdr.Cap = nbElems
+		return nil
+	}
+
+	size, err := isPP(vpp)
+	if err != nil {
+		return err
+	}
+	p := vpp.Elem()
+	t := p.Type().Elem()
+	if size > len(*s) {
+		return wrapf("can't map struct %s (size %d) on [%d]byte", t, size, len(*s))
+	}
+	// Use casting black magic to read the internal slice headers.
+	dest := unsafe.Pointer(((*reflect.SliceHeader)(unsafe.Pointer(s))).Data)
+	// Use reflection black magic to write to the original pointer.
+	p.Set(reflect.NewAt(t, dest))
+	return nil
+}
+
+// View represents a view of physical memory memory mapped into user space.
+//
+// It is usually used to map CPU registers into user space, usually I/O
+// registers and the likes.
+//
+// It is not required to call Close(), the kernel will clean up on process
+// shutdown.
+type View struct {
+	Slice
+	orig []uint8 // Reference rounded to the lowest 4Kb page containing Slice.
+	phys uint64  // physical address of the base of Slice.
+}
+
+// Close unmaps the memory from the user address space.
+//
+// This is done naturally by the OS on process teardown (when the process
+// exits) so this is not a hard requirement to call this function.
+func (v *View) Close() error {
+	return munmap(v.orig)
+}
+
+// PhysAddr implements Mem.
+func (v *View) PhysAddr() uint64 {
+	return v.phys
+}
+
+// MapGPIO returns a CPU specific memory mapping of the CPU I/O registers using
+// /dev/gpiomem.
+//
+// At the moment, /dev/gpiomem is only supported on Raspbian Jessie via a
+// specific kernel driver.
+func MapGPIO() (*View, error) {
+	if isLinux {
+		return mapGPIOLinux()
+	}
+	return nil, wrapf("/dev/gpiomem is not supported on this platform")
+}
+
+// Map returns a memory mapped view of arbitrary physical memory range using OS
+// provided functionality.
+//
+// Maps size of memory, rounded on a 4kb window.
+//
+// This function is dangerous and should be used wisely. It normally requires
+// super privileges (root). On Linux, it leverages /dev/mem.
+func Map(base uint64, size int) (*View, error) {
+	if isLinux {
+		return mapLinux(base, size)
+	}
+	return nil, wrapf("physical memory mapping is not supported on this platform")
+}
+
+// MapAsPOD is a leaky shorthand of calling Map(base, sizeof(v)) then AsPOD(v).
+//
+// There is no way to reclaim the memory map.
+//
+// A slice cannot be used, as it does not have inherent size. Use an aray
+// instead.
+func MapAsPOD(base uint64, i interface{}) error {
+	// Automatically determine the necessary size. Because of this, slice of
+	// unspecified length cannot be used here.
+	if i == nil {
+		return wrapf("require Ptr, got nil")
+	}
+	v := reflect.ValueOf(i)
+	size, err := isPP(v)
+	if err != nil {
+		return err
+	}
+	m, err := Map(base, size)
+	if err != nil {
+		return err
+	}
+	return m.AsPOD(i)
+}
+
+//
+
+// Keep a cache of open file handles instead of opening and closing repeatedly.
+var (
+	mu          sync.Mutex
+	gpioMemErr  error
+	gpioMemView *View
+	devMem      fileIO
+	devMemErr   error
+	openFile    = openFileOrig
+)
+
+type fileIO interface {
+	io.Closer
+	io.Seeker
+	io.Reader
+	Fd() uintptr
+}
+
+func openFileOrig(path string, flag int) (fileIO, error) {
+	f, err := fs.Open(path, flag)
+	if err != nil {
+		return nil, err
+	}
+	return f, nil
+}
+
+// mapGPIOLinux is purely Raspbian specific.
+func mapGPIOLinux() (*View, error) {
+	mu.Lock()
+	defer mu.Unlock()
+	if gpioMemView == nil && gpioMemErr == nil {
+		if f, err := openFile("/dev/gpiomem", os.O_RDWR|os.O_SYNC); err == nil {
+			defer f.Close()
+			if i, err := mmap(f.Fd(), 0, pageSize); err == nil {
+				gpioMemView = &View{Slice: i, orig: i, phys: 0}
+			} else {
+				gpioMemErr = wrapf("failed to memory map in user space GPIO memory: %v", err)
+			}
+		} else {
+			gpioMemErr = wrapf("failed to open GPIO memory: %v", err)
+		}
+	}
+	return gpioMemView, gpioMemErr
+}
+
+// mapLinux leverages /dev/mem to map a view of physical memory.
+func mapLinux(base uint64, size int) (*View, error) {
+	f, err := openDevMemLinux()
+	if err != nil {
+		return nil, err
+	}
+	// Align base and size at 4Kb.
+	offset := int(base & 0xFFF)
+	i, err := mmap(f.Fd(), int64(base&^0xFFF), (size+offset+0xFFF)&^0xFFF)
+	if err != nil {
+		return nil, wrapf("mapping at 0x%x failed: %v", base, err)
+	}
+	return &View{Slice: i[offset : offset+size], orig: i, phys: base + uint64(offset)}, nil
+}
+
+func openDevMemLinux() (fileIO, error) {
+	mu.Lock()
+	defer mu.Unlock()
+	if devMem == nil && devMemErr == nil {
+		if devMem, devMemErr = openFile("/dev/mem", os.O_RDWR|os.O_SYNC); devMemErr != nil {
+			devMemErr = wrapf("failed to open physical memory: %v", devMemErr)
+		}
+	}
+	return devMem, devMemErr
+}
+
+func isAcceptableInner(t reflect.Type) error {
+	switch k := t.Kind(); k {
+	case reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
+		reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64,
+		reflect.Float32, reflect.Float64:
+		return nil
+	case reflect.Array:
+		return isAcceptableInner(t.Elem())
+	case reflect.Struct:
+		for i := 0; i < t.NumField(); i++ {
+			if err := isAcceptableInner(t.Field(i).Type); err != nil {
+				return err
+			}
+		}
+		return nil
+	default:
+		return wrapf("require Ptr to Ptr to a POD type, got Ptr to Ptr to %s", k)
+	}
+}
+
+// isPP makes sure it is a pointer to a nil-pointer to something. It does
+// sanity checks to reduce likelihood of a panic().
+func isPP(pp reflect.Value) (int, error) {
+	if k := pp.Kind(); k != reflect.Ptr {
+		return 0, wrapf("require Ptr, got %s of %s", k, pp.Type().Name())
+	}
+	p := pp.Elem()
+	if k := p.Kind(); k != reflect.Ptr {
+		return 0, wrapf("require Ptr to Ptr, got %s", k)
+	}
+	if !p.IsNil() {
+		return 0, wrapf("require Ptr to Ptr to be nil")
+	}
+	// p.Elem() can't be used since it's a nil pointer. Use the type instead.
+	t := p.Type().Elem()
+	if err := isAcceptableInner(t); err != nil {
+		return 0, err
+	}
+	return int(t.Size()), nil
+}
+
+// isPS makes sure it is a pointer to a nil-slice of something. It does
+// sanity checks to reduce likelihood of a panic().
+func isPS(bufSize int, ps reflect.Value) (int, error) {
+	if k := ps.Kind(); k != reflect.Ptr {
+		return 0, wrapf("require Ptr, got %s of %s", k, ps.Type().Name())
+	}
+	s := ps.Elem()
+	if k := s.Kind(); k != reflect.Slice {
+		return 0, wrapf("require Ptr to Slice, got %s", k)
+	}
+	if !s.IsNil() {
+		return 0, wrapf("require Ptr to Slice to be nil")
+	}
+	// s.Elem() can't be used since it's a nil slice. Use the type instead.
+	t := s.Type().Elem()
+	if err := isAcceptableInner(t); err != nil {
+		return 0, err
+	}
+	return int(t.Size()), nil
+}
+
+func wrapf(format string, a ...interface{}) error {
+	return fmt.Errorf("pmem: "+format, a...)
+}