One way of supporting a guard page below kstack: allocate kstacks in

procinit() and map them high up (below TRAMPOLNE) with an empty
mapping below each stack.  Never free a kernel stack.

Another way would be to allocate and map them dynamically, but then we
need to reload page table when switching processes in scheduler()
and/or have a kernel pagetable per proc (if we want k->stack to be the
same virtual address in each process).

One gotcha: kernel addresses are not equal to physical addresses for
stack addresses.  A stack address must be translated if we need its
physical address (e.g., virtio passes a stack address to the disk).

kernel/defs.h

@@ -193,6 +193,8 @@ uint64          walkaddr(pagetable_t, uint64);
 int             copyout(pagetable_t, uint64, char *, uint64);
 int             copyin(pagetable_t, char *, uint64, uint64);
 int             copyinstr(pagetable_t pagetable, char *dst, uint64 srcva, uint64 max);
+char*           map_kstack();
+uint64          kernelpa(uint64);
 
 // plic.c
 void            plicinit(void);

kernel/memlayout.h

@@ -53,3 +53,4 @@
 // map the trampoline page to the highest address,
 // in both user and kernel space.
 #define TRAMPOLINE (MAXVA - PGSIZE)
+#define KSTACK(p) ((TRAMPOLINE-PGSIZE)-p*2*PGSIZE)

kernel/proc.c

@@ -26,8 +26,14 @@ procinit(void)
   struct proc *p;
   
   initlock(&pid_lock, "nextpid");
-  for(p = proc; p < &proc[NPROC]; p++)
+  for(p = proc; p < &proc[NPROC]; p++) {
       initlock(&p->lock, "proc");
+      // Allocate a page for the kernel stack.
+      char *kstack = (char *) KSTACK((int) (p - proc));
+      if((p->kstack = map_kstack(kstack)) == 0) {
+        panic("procinit");
+      }
+  }
 }
 
 // Must be called with interrupts disabled,
@@ -94,16 +100,8 @@ allocproc(void)
 found:
   p->pid = allocpid();
 
-  // Allocate a page for the kernel stack.
-  if((p->kstack = kalloc()) == 0){
-    release(&p->lock);
-    return 0;
-  }
-
   // Allocate a trapframe page.
   if((p->tf = (struct trapframe *)kalloc()) == 0){
-    kfree(p->kstack);
-    p->kstack = 0;
     release(&p->lock);
     return 0;
   }
@@ -126,9 +124,6 @@ found:
 static void
 freeproc(struct proc *p)
 {
-  if(p->kstack)
-    kfree(p->kstack);
-  p->kstack = 0;
   if(p->tf)
     kfree((void*)p->tf);
   p->tf = 0;
@@ -651,4 +646,3 @@ procdump(void)
     printf("\n");
   }
 }
-

kernel/virtio_disk.c

@@ -199,7 +199,7 @@ virtio_disk_rw(struct buf *b)
   buf0.reserved = 0;
   buf0.sector = sector;
 
-  desc[idx[0]].addr = (uint64) &buf0;
+  desc[idx[0]].addr = (uint64) kernelpa((uint64) &buf0);
   desc[idx[0]].len = sizeof(buf0);
   desc[idx[0]].flags = VRING_DESC_F_NEXT;
   desc[idx[0]].next = idx[1];

kernel/vm.c

@@ -404,3 +404,34 @@ copyinstr(pagetable_t pagetable, char *dst, uint64 srcva, uint64 max)
     return -1;
   }
 }
+
+char *map_kstack(uint64 kstack)
+{
+  char *k = kalloc();
+  if(k == 0) {
+    return 0;
+  }
+  if (mappages(kernel_pagetable, (uint64) kstack, PGSIZE,
+               (uint64) k, PTE_R | PTE_W) == 0) {
+    kvminithart();
+    return (char *) kstack;
+  }
+  kfree(k);
+  return 0;
+}
+
+// assumes va is page aligned
+uint64
+kernelpa(uint64 va) {
+  uint64 off = va % PGSIZE;
+  pte_t *pte;
+  uint64 pa;
+  
+  pte = walk(kernel_pagetable, va, 0);
+  if(pte == 0)
+    panic("kernelpa");
+  if((*pte & PTE_V) == 0)
+    panic("kernelpa");
+  pa = PTE2PA(*pte);
+  return pa+off;
+}