Add support for virtual -> physical address translation

kernel/mem/vmm/pte.go

@@ -1,10 +1,16 @@
 package vmm
 
 import (
+	"github.com/achilleasa/gopher-os/kernel"
 	"github.com/achilleasa/gopher-os/kernel/mem"
 	"github.com/achilleasa/gopher-os/kernel/mem/pmm"
 )
 
+var (
+	// ErrInvalidMapping is returned when trying to lookup a virtual memory address that is not yet mapped.
+	ErrInvalidMapping = &kernel.Error{Module: "vmm", Message: "virtual address does not point to a mapped physical page"}
+)
+
 // PageTableEntryFlag describes a flag that can be applied to a page table entry.
 type PageTableEntryFlag uintptr
 
@@ -42,3 +48,27 @@ func (pte pageTableEntry) Frame() pmm.Frame {
 func (pte *pageTableEntry) SetFrame(frame pmm.Frame) {
 	*pte = (pageTableEntry)((uintptr(*pte) &^ ptePhysPageMask) | frame.Address())
 }
+
+// pteForAddress returns the final page table entry that correspond to a
+// particular virtual address. The function performs a page table walk till it
+// reaches the final page table entry returning ErrInvalidMapping if the page
+// is not present.
+func pteForAddress(virtAddr uintptr) (*pageTableEntry, *kernel.Error) {
+	var (
+		err   *kernel.Error
+		entry *pageTableEntry
+	)
+
+	walk(virtAddr, func(pteLevel uint8, pte *pageTableEntry) bool {
+		if !pte.HasFlags(FlagPresent) {
+			entry = nil
+			err = ErrInvalidMapping
+			return false
+		}
+
+		entry = pte
+		return true
+	})
+
+	return entry, err
+}

kernel/mem/vmm/translate.go

@@ -0,0 +1,19 @@
+package vmm
+
+import "github.com/achilleasa/gopher-os/kernel"
+
+// Translate returns the physical address that corresponds to the supplied
+// virtual address or ErrInvalidMapping if the virtual address does not
+// correspond to a mapped physical address.
+func Translate(virtAddr uintptr) (uintptr, *kernel.Error) {
+	pte, err := pteForAddress(virtAddr)
+	if err != nil {
+		return 0, err
+	}
+
+	// Calculate the physical address by taking the physical frame address and
+	// appending the offset from the virtual address
+	physAddr := pte.Frame().Address() + (virtAddr & ((1 << pageLevelShifts[pageLevels-1]) - 1))
+
+	return physAddr, nil
+}

kernel/mem/vmm/translate_test.go

@@ -0,0 +1,73 @@
+package vmm
+
+import (
+	"runtime"
+	"testing"
+	"unsafe"
+
+	"github.com/achilleasa/gopher-os/kernel/mem/pmm"
+)
+
+func TestTranslateAmd64(t *testing.T) {
+	if runtime.GOARCH != "amd64" {
+		t.Skip("test requires amd64 runtime; skipping")
+	}
+
+	defer func(origPtePtr func(uintptr) unsafe.Pointer) {
+		ptePtrFn = origPtePtr
+	}(ptePtrFn)
+
+	// the virtual address just contains the page offset
+	virtAddr := uintptr(1234)
+	expFrame := pmm.Frame(42)
+	expPhysAddr := expFrame.Address() + virtAddr
+	specs := [][pageLevels]bool{
+		{true, true, true, true},
+		{false, true, true, true},
+		{true, false, true, true},
+		{true, true, false, true},
+		{true, true, true, false},
+	}
+
+	for specIndex, spec := range specs {
+		pteCallCount := 0
+		ptePtrFn = func(entry uintptr) unsafe.Pointer {
+			var pte pageTableEntry
+			pte.SetFrame(expFrame)
+			if specs[specIndex][pteCallCount] {
+				pte.SetFlags(FlagPresent)
+			}
+			pteCallCount++
+
+			return unsafe.Pointer(&pte)
+		}
+
+		// An error is expected if any page level contains a non-present page
+		expError := false
+		for _, hasMapping := range spec {
+			if !hasMapping {
+				expError = true
+				break
+			}
+		}
+
+		physAddr, err := Translate(virtAddr)
+		switch {
+		case expError && err != ErrInvalidMapping:
+			t.Errorf("[spec %d] expected to get ErrInvalidMapping; got %v", specIndex, err)
+		case !expError && err != nil:
+			t.Errorf("[spec %d] unexpected error %v", specIndex, err)
+		case !expError && physAddr != expPhysAddr:
+			t.Errorf("[spec %d] expected phys addr to be 0x%x; got 0x%x", specIndex, expPhysAddr, physAddr)
+		}
+	}
+}
+
+/*
+	phys, err := vmm.Translate(uintptr(100 * mem.Mb))
+	if err != nil {
+		early.Printf("err: %s\n", err.Error())
+	} else {
+		early.Printf("phys: 0x%x\n", phys)
+	}
+*/