From b67a2045b9470123e20b83ec3017758a490063fe Mon Sep 17 00:00:00 2001
From: Achilleas Anagnostopoulos <achilleas.imap.mbox@gmail.com>
Date: Wed, 31 May 2017 16:02:55 +0100
Subject: [PATCH] Define virtual memory page type

This is equivalent to pmm.Frame (also a uintptr) but having different
types for physical and virtual frames serves as an additional layer of
protection for functions/methods that receive physical and/or virtual
page arguments.
---
 kernel/mem/vmm/page.go      | 19 +++++++++++++++++++
 kernel/mem/vmm/page_test.go | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)
 create mode 100644 kernel/mem/vmm/page.go
 create mode 100644 kernel/mem/vmm/page_test.go

diff --git a/kernel/mem/vmm/page.go b/kernel/mem/vmm/page.go
new file mode 100644
index 0000000..f1e9cb4
--- /dev/null
+++ b/kernel/mem/vmm/page.go
@@ -0,0 +1,19 @@
+package vmm
+
+import "github.com/achilleasa/gopher-os/kernel/mem"
+
+// Page describes a virtual memory page index.
+type Page uintptr
+
+// Address returns a pointer to the virtual memory address pointed to by this Page.
+func (f Page) Address() uintptr {
+	return uintptr(f << mem.PageShift)
+}
+
+// PageFromAddress returns a Page that corresponds to the given virtual
+// address. This function can handle both page-aligned and not aligned virtual
+// addresses. in the latter case, the input address will be rounded down to the
+// page that contains it.
+func PageFromAddress(virtAddr uintptr) Page {
+	return Page((virtAddr & ^(uintptr(mem.PageSize - 1))) >> mem.PageShift)
+}
diff --git a/kernel/mem/vmm/page_test.go b/kernel/mem/vmm/page_test.go
new file mode 100644
index 0000000..8786e97
--- /dev/null
+++ b/kernel/mem/vmm/page_test.go
@@ -0,0 +1,35 @@
+package vmm
+
+import (
+	"testing"
+
+	"github.com/achilleasa/gopher-os/kernel/mem"
+)
+
+func TestPageMethods(t *testing.T) {
+	for pageIndex := uint64(0); pageIndex < 128; pageIndex++ {
+		page := Page(pageIndex)
+
+		if exp, got := uintptr(pageIndex<<mem.PageShift), page.Address(); got != exp {
+			t.Errorf("expected page (%d, index: %d) call to Address() to return %x; got %x", page, pageIndex, exp, got)
+		}
+	}
+}
+
+func TestPageFromAddress(t *testing.T) {
+	specs := []struct {
+		input   uintptr
+		expPage Page
+	}{
+		{0, Page(0)},
+		{4095, Page(0)},
+		{4096, Page(1)},
+		{4123, Page(1)},
+	}
+
+	for specIndex, spec := range specs {
+		if got := PageFromAddress(spec.input); got != spec.expPage {
+			t.Errorf("[spec %d] expected returned page to be %v; got %v", specIndex, spec.expPage, got)
+		}
+	}
+}