diff --git a/kernel/proc.c b/kernel/proc.c
index 0655783..6ba3fec 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -544,7 +544,7 @@ sleep(void *chan, struct spinlock *lk)
 }
 
 //PAGEBREAK!
-// Wake up p, used by exit().
+// Wake up p if it is sleeping in wait(); used by exit().
 // Caller must hold p->lock.
 static void
 wakeup1(struct proc *p)
diff --git a/kernel/riscv.h b/kernel/riscv.h
index e5c0f64..e35f3bc 100644
--- a/kernel/riscv.h
+++ b/kernel/riscv.h
@@ -312,6 +312,17 @@ r_ra()
   return x;
 }
 
+// tell the machine to finish any previous writes to
+// PTEs, so that a subsequent use of a virtual
+// address or load of the SATP will see those writes.
+// perhaps this also flushes the TLB.
+static inline void
+sfence_vma()
+{
+  // the zero, zero means flush all TLB entries.
+  asm volatile("sfence.vma zero, zero");
+}
+
 
 #define PGSIZE 4096 // bytes per page
 #define PGSHIFT 12  // bits of offset within a page
diff --git a/kernel/trampoline.S b/kernel/trampoline.S
index b992ea6..471a29c 100644
--- a/kernel/trampoline.S
+++ b/kernel/trampoline.S
@@ -17,7 +17,8 @@ trampout:
 	# a0: p->tf in user page table
         # a1: new value for satp, for user page table
 
-        # switch to user page table
+	# switch to user page table.
+	sfence.vma zero, zero
         csrw satp, a1
 
         # put the saved user a0 in sscratch, so we
@@ -128,6 +129,7 @@ trampin:
 
         # restore kernel page table from p->tf->kernel_satp
         ld t1, 0(a0)
+	sfence.vma zero, zero
         csrw satp, t1
 
         # a0 is no longer valid, since the kernel page
diff --git a/kernel/vm.c b/kernel/vm.c
index bdb53c2..412ec8c 100644
--- a/kernel/vm.c
+++ b/kernel/vm.c
@@ -61,6 +61,7 @@ kvminit()
 void
 kvminithart()
 {
+  sfence_vma();
   w_satp(MAKE_SATP(kernel_pagetable));
 }