The start of SYSCALL and SYSRET.

709c2160 · Silas Boyd-Wickizer · f25a83b9 · 709c2160 · 709c2160 · 709c2160
--- a/mmu.h
+++ b/mmu.h
@@ -57,8 +57,8 @@ struct segdesc {
 #define KCSEG   (2<<3)  /* kernel code segment */
 #define KDSEG   (3<<3)  /* kernel data segment */
 #define TSSSEG  (4<<3)  /* tss segment - takes two slots */
-#define UCSEG   (6<<3)  /* user code segment */
-#define UDSEG   (7<<3)  /* user data segment */
+#define UDSEG   (6<<3)  /* user data segment */
+#define UCSEG   (7<<3)  /* user code segment */
 #define NSEGS   8

 // User segment bits (SEG_S set).

--- a/trap.c
+++ b/trap.c
@@ -169,8 +169,10 @@ inittrap(void)
 void
 initseg(void)
 {
+  extern void sysentry(void);
  volatile struct desctr dtr;
  struct cpu *c;
+  u64 star;

  dtr.limit = sizeof(idt) - 1;
  dtr.base = (u64)idt;
@@ -183,4 +185,13 @@ initseg(void)
  dtr.limit = sizeof(c->gdt) - 1;
  dtr.base = (u64)c->gdt;
  lgdt((void *)&dtr.limit);
+
+  // When executing a syscall instruction the CPU sets the SS selector
+  // to (star >> 32) + 8 and the CS selector to (star >> 32).
+  // When executing a sysret instruction the CPU sets the SS selector
+  // to (star >> 48) + 8 and the CS selector to (star >> 48) + 16.
+  star = ((((u64)UCSEG|0x3) - 16)<<48)|((u64)KCSEG<<32);
+  writemsr(MSR_STAR, star);
+  writemsr(MSR_LSTAR, (u64)&sysentry);
+  writemsr(MSR_SFMASK, FL_TF);
 }
--- a/trapasm.S
+++ b/trapasm.S
@@ -14,6 +14,11 @@
 #define TRAPCODE(x) _TRAP(x, trapcommon)

 .code64
+.globl sysentry
+.align 8
+sysentry:
+        jmp sysentry
+
 .globl trappush
 .align 8
 trappush: