From 61314a9c33176a11decbe175d8e593bc8950815f Mon Sep 17 00:00:00 2001
From: Achilleas Anagnostopoulos <achilleas.imap.mbox@gmail.com>
Date: Wed, 10 May 2017 06:59:48 +0100
Subject: [PATCH] Define memory Size and implement optimized memset

---
 kernel/mem/constants_amd64.go | 13 +++++++++++++
 kernel/mem/memset.go          | 29 +++++++++++++++++++++++++++++
 kernel/mem/memset_test.go     | 27 +++++++++++++++++++++++++++
 kernel/mem/size.go            | 12 ++++++++++++
 4 files changed, 81 insertions(+)
 create mode 100644 kernel/mem/constants_amd64.go
 create mode 100644 kernel/mem/memset.go
 create mode 100644 kernel/mem/memset_test.go
 create mode 100644 kernel/mem/size.go

diff --git a/kernel/mem/constants_amd64.go b/kernel/mem/constants_amd64.go
new file mode 100644
index 0000000..aea2911
--- /dev/null
+++ b/kernel/mem/constants_amd64.go
@@ -0,0 +1,13 @@
+// +build amd64
+
+package mem
+
+const (
+	// PageShift is equal to log2(PageSize). This constant is used when
+	// we need to convert a physical address to a page number (shift right by PageShift)
+	// and vice-versa.
+	PageShift = 12
+
+	// PageSize defines the system's page size in bytes.
+	PageSize = Size(1 << PageShift)
+)
diff --git a/kernel/mem/memset.go b/kernel/mem/memset.go
new file mode 100644
index 0000000..6041487
--- /dev/null
+++ b/kernel/mem/memset.go
@@ -0,0 +1,29 @@
+package mem
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+// Memset sets size bytes at the given address to the supplied value. The implementation
+// is based on bytes.Repeat; instead of using a for loop, this function uses
+// log2(size) copy calls which should give us a speed boost as page addresses
+// are always aligned.
+func Memset(addr uintptr, value byte, size Size) {
+	if size == 0 {
+		return
+	}
+
+	// overlay a slice on top of this address region
+	target := *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
+		Len:  int(size),
+		Cap:  int(size),
+		Data: addr,
+	}))
+
+	// Set first element and make log2(size) optimized copies
+	target[0] = value
+	for index := Size(1); index < size; index *= 2 {
+		copy(target[index:], target[:index])
+	}
+}
diff --git a/kernel/mem/memset_test.go b/kernel/mem/memset_test.go
new file mode 100644
index 0000000..c4af743
--- /dev/null
+++ b/kernel/mem/memset_test.go
@@ -0,0 +1,27 @@
+package mem
+
+import (
+	"testing"
+	"unsafe"
+)
+
+func TestMemset(t *testing.T) {
+	// memset with a 0 size should be a no-op
+	Memset(uintptr(0), 0x00, 0)
+
+	for pageCount := uint32(1); pageCount <= 10; pageCount++ {
+		buf := make([]byte, PageSize<<pageCount)
+		for i := 0; i < len(buf); i++ {
+			buf[i] = 0xFE
+		}
+
+		addr := uintptr(unsafe.Pointer(&buf[0]))
+		Memset(addr, 0x00, Size(len(buf)))
+
+		for i := 0; i < len(buf); i++ {
+			if got := buf[i]; got != 0x00 {
+				t.Errorf("[block with %d pages] expected byte: %d to be 0x00; got 0x%x", pageCount, i, got)
+			}
+		}
+	}
+}
diff --git a/kernel/mem/size.go b/kernel/mem/size.go
new file mode 100644
index 0000000..626d4d5
--- /dev/null
+++ b/kernel/mem/size.go
@@ -0,0 +1,12 @@
+package mem
+
+// Size represents a memory block size in bytes.
+type Size uint64
+
+// Common memory block sizes.
+const (
+	Byte Size = 1
+	Kb        = 1024 * Byte
+	Mb        = 1024 * Kb
+	Gb        = 1024 * Mb
+)