Minor ring0 interface cleanup.

- Remove unused methods.
- Provide declaration for asm function.

PiperOrigin-RevId: 200146850
Change-Id: Ic455c96ffe0d2e78ef15f824eb65d7de705b054a

pkg/sentry/platform/kvm/machine.go

@@ -40,7 +40,7 @@ type machine struct {
 	nextSlot uint32
 
 	// kernel is the set of global structures.
-	kernel *ring0.Kernel
+	kernel ring0.Kernel
 
 	// mappingCache is used for mapPhysical.
 	mappingCache sync.Map
@@ -135,7 +135,7 @@ func newMachine(vm int, vCPUs int) (*machine, error) {
 		// issues when you've got > n active threads.)
 		vCPUs = n
 	}
-	m.kernel = ring0.New(ring0.KernelOpts{
+	m.kernel.Init(ring0.KernelOpts{
 		PageTables: pagetables.New(newAllocator()),
 	})
 
@@ -158,7 +158,7 @@ func newMachine(vm int, vCPUs int) (*machine, error) {
 			fd:      int(fd),
 			machine: m,
 		}
-		c.CPU.Init(m.kernel)
+		c.CPU.Init(&m.kernel)
 		c.CPU.KernelSyscall = bluepillSyscall
 		c.CPU.KernelException = bluepillException
 		m.vCPUs[uint64(-id)] = c // See above.

pkg/sentry/platform/ring0/kernel.go

@@ -14,27 +14,13 @@
 
 package ring0
 
-// New creates a new kernel.
+// Init initializes a new kernel.
 //
 // N.B. that constraints on KernelOpts must be satisfied.
 //
-// Init must have been called.
-func New(opts KernelOpts) *Kernel {
-	k := new(Kernel)
+//go:nosplit
+func (k *Kernel) Init(opts KernelOpts) {
 	k.init(opts)
-	return k
-}
-
-// NewCPU creates a new CPU associated with this Kernel.
-//
-// Note that execution of the new CPU must begin at Start, with constraints as
-// documented. Initialization is not completed by this method alone.
-//
-// See also Init.
-func (k *Kernel) NewCPU() *CPU {
-	c := new(CPU)
-	c.Init(k)
-	return c
 }
 
 // Halt halts execution.
@@ -56,8 +42,7 @@ func defaultSyscall() { Halt() }
 //go:nosplit
 func defaultException(Vector) { Halt() }
 
-// Init allows the initialization of a CPU from a kernel without allocation.
-// The same constraints as NewCPU apply.
+// Init initializes a new CPU.
 //
 // Init allows embedding in other objects.
 func (c *CPU) Init(k *Kernel) {

pkg/sentry/platform/ring0/lib_amd64.go

@@ -64,6 +64,9 @@ func wrgsmsr(addr uintptr)
 // writeCR3 writes the CR3 value.
 func writeCR3(phys uintptr)
 
+// readCR3 reads the current CR3 value.
+func readCR3() uintptr
+
 // readCR2 reads the current CR2 value.
 func readCR2() uintptr