User swapgs to load the correct GS_BASE

bae70a10 · Silas Boyd-Wickizer · b40a7ea0 · bae70a10 · bae70a10 · bae70a10
--- a/kernel/asmdefines.c
+++ b/kernel/asmdefines.c
@@ -12,5 +12,6 @@ void
 asmdefines(void)
 {
  DEFINE(PROC_KSTACK_OFFSET, __offsetof(struct proc, kstack));
+  DEFINE(TF_CS, __offsetof(struct trapframe, cs));
  DEFINE(TRAPFRAME_SIZE, sizeof(trapframe));
 }
--- a/kernel/trap.cc
+++ b/kernel/trap.cc
@@ -20,9 +20,6 @@ extern u64 trapentry[];
 u64
 sysentry_c(u64 a0, u64 a1, u64 a2, u64 a3, u64 a4, u64 num)
 {
-  writegs(KDSEG);
-  writemsr(MSR_GS_BASE, (u64)&cpus[lapicid()].cpu);
-
  sti();

  if(myproc()->killed) {
@@ -45,9 +42,6 @@ sysentry_c(u64 a0, u64 a1, u64 a2, u64 a3, u64 a4, u64 num)
 void
 trap(struct trapframe *tf)
 {
-  writegs(KDSEG);
-  writemsr(MSR_GS_BASE, (u64)&cpus[lapicid()].cpu);
-
  if (tf->trapno == T_NMI) {
    // The only locks that we can acquire during NMI are ones
    // we acquire only during NMI.

--- a/kernel/trapasm.S
+++ b/kernel/trapasm.S
@@ -28,7 +28,6 @@ sysentry:
        // blow away %r9: syscalls can take at most 5 args
        swapgs
        movq    %gs:8, %r9  // myproc()
-        swapgs

        movq    %ss:PROC_KSTACK_OFFSET(%r9), %r9
        addq    $(KSTACKSIZE-TRAPFRAME_SIZE), %r9
@@ -72,6 +71,8 @@ sysentry:
        // return using SYSRET
        cli
        movq    %rsp, %r11
+        swapgs
+
        movw    $UDSEG, %cx
        movw    %cx, %ds
        movw    %cx, %es
@@ -110,9 +111,12 @@ trapcommon:
        movw $KDSEG, %ax
        movw %ax, %ds
        movw %ax, %es
-        // XXX(sbw) we should do something with fs, gs, gs.base

-        movq %rsp, %rdi  // first argument to trap
+        cmp  $KCSEG, TF_CS(%rsp)
+        jz 1f
+        swapgs
+
+1:      movq %rsp, %rdi  // first argument to trap
        xor %rbp, %rbp
        call trap
        // Fall through to trapret
@@ -120,11 +124,15 @@ trapcommon:
 .globl trapret
 .align 8
 trapret:
-        addq $0xe, %rsp  // padding
+        cli
+        cmp $KCSEG, TF_CS(%rsp)
+        jz 1f
+        swapgs
+
+1:      addq $0xe, %rsp  // padding
        popw %ax
        movw %ax, %ds
        movw %ax, %es
-        // XXX(sbw) we should do something with fs, gs, gs.base
        
        popq %r15
        popq %r14