Merge branch 'riscv' into util

4 files changed, 22 insertions, 2 deletions

diff --git a/kernel/fs.c b/kernel/fs.c
index dea5864..c6bab15 100644
--- a/kernel/fs.c
+++ b/kernel/fs.c
@@ -194,7 +194,7 @@ static struct inode* iget(uint dev, uint inum);
 // Allocate an inode on device dev.
 // Mark it as allocated by  giving it type type.
 // Returns an unlocked but allocated and referenced inode,
-// or NULL if there is no free inode..
+// or NULL if there is no free inode.
 struct inode*
 ialloc(uint dev, short type)
 {
diff --git a/kernel/syscall.c b/kernel/syscall.c
index ee94696..ed65409 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -79,6 +79,7 @@ argstr(int n, char *buf, int max)
   return fetchstr(addr, buf, max);
 }
 
+// Prototypes for the functions that handle system calls.
 extern uint64 sys_fork(void);
 extern uint64 sys_exit(void);
 extern uint64 sys_wait(void);
@@ -101,6 +102,8 @@ extern uint64 sys_link(void);
 extern uint64 sys_mkdir(void);
 extern uint64 sys_close(void);
 
+// An array mapping syscall numbers from syscall.h
+// to the function that handles the system call.
 static uint64 (*syscalls[])(void) = {
 [SYS_fork]    sys_fork,
 [SYS_exit]    sys_exit,
@@ -133,6 +136,8 @@ syscall(void)
 
   num = p->trapframe->a7;
   if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {
+    // Use num to lookup the system call function for num, call it,
+    // and store its return value in p->trapframe->a0
     p->trapframe->a0 = syscalls[num]();
   } else {
     printf("%d %s: unknown sys call %d\n",
diff --git a/kernel/trampoline.S b/kernel/trampoline.S
index 0aaa413..d7308cc 100644
--- a/kernel/trampoline.S
+++ b/kernel/trampoline.S
@@ -80,9 +80,18 @@ uservec:
         # load the address of usertrap(), from p->trapframe->kernel_trap
         ld t0, 16(a0)
 
-        # load the kernel page table, from p->trapframe->kernel_satp
+
+        # fetch the kernel page table address, from p->trapframe->kernel_satp.
         ld t1, 0(a0)
+
+        # wait for any previous memory operations to complete, so that
+        # they use the user page table.
+        sfence.vma zero, zero
+
+        # install the kernel page table.
         csrw satp, t1
+
+        # flush now-stale user entries from the TLB.
         sfence.vma zero, zero
 
         # jump to usertrap(), which does not return
@@ -96,6 +105,7 @@ userret:
         # a0: user page table, for satp.
 
         # switch to the user page table.
+        sfence.vma zero, zero
         csrw satp, a0
         sfence.vma zero, zero
 
diff --git a/kernel/vm.c b/kernel/vm.c
index 284b72d..9f69783 100644
--- a/kernel/vm.c
+++ b/kernel/vm.c
@@ -61,7 +61,12 @@ kvminit(void)
 void
 kvminithart()
 {
+  // wait for any previous writes to the page table memory to finish.
+  sfence_vma();
+
   w_satp(MAKE_SATP(kernel_pagetable));
+
+  // flush stale entries from the TLB.
   sfence_vma();
 }