add some comments

find out the hard way why user and kernel must have separate segment descriptors

1 files changed, 6 insertions, 3 deletions

diff --git a/vm.c b/vm.c
index c94e9a3..9ea5e92 100644
--- a/vm.c
+++ b/vm.c
@@ -93,12 +93,15 @@ ksegment(void)
 {
   struct cpu *c;
 
-  // Map once virtual addresses to linear addresses using identity map
+  // Map virtual addresses to linear addresses using identity map.
+  // Cannot share a CODE descriptor for both kernel and user
+  // because it would have to have DPL_USR, but the CPU forbids
+  // an interrupt from CPL=0 to DPL=3.
   c = &cpus[cpunum()];
   c->gdt[SEG_KCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, 0);
   c->gdt[SEG_KDATA] = SEG(STA_W, 0, 0xffffffff, 0);
-  c->gdt[SEG_UCODE] = SEG(STA_X|STA_R, 0x0, 0xffffffff, DPL_USER);
-  c->gdt[SEG_UDATA] = SEG(STA_W, 0x0, 0xffffffff, DPL_USER);
+  c->gdt[SEG_UCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, DPL_USER);
+  c->gdt[SEG_UDATA] = SEG(STA_W, 0, 0xffffffff, DPL_USER);
 
   // map cpu, and curproc
   c->gdt[SEG_KCPU] = SEG(STA_W, &c->cpu, 8, 0);