x86/asm/entry/64: Fix comments

[deliverable/linux.git] / arch / x86 / kernel / entry_64.S

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index e8372e08f8c2512a0b21a51d2b9c6e4ffda63f41..695f4d434a847d02b15f38b0503639e8ea963b7d 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -14,9 +14,6 @@
  * NOTE: This code handles signal-recognition, which happens every time
  * after an interrupt and after each system call.
  *
- * Normal syscalls and interrupts don't save a full stack frame, this is
- * only done for syscall tracing, signals or fork/exec et.al.
- *
  * A note on terminology:
  * - top of stack: Architecture defined interrupt frame from SS to RIP
  * at the top of the kernel process stack.
@@ -151,7 +148,7 @@ ENDPROC(native_usergs_sysret64)
        .endm
 
 /*
- * initial frame state for interrupts (and exceptions without error code)
+ * empty frame
  */
        .macro EMPTY_FRAME start=1 offset=0
        .if \start
@@ -379,7 +376,7 @@ tracesys_phase2:
        call syscall_trace_enter_phase2
 
        /*
-        * Reload arg registers from stack in case ptrace changed them.
+        * Reload registers from stack in case ptrace changed them.
         * We don't reload %rax because syscall_trace_entry_phase2() returned
         * the value it wants us to use in the table lookup.
         */
@@ -629,6 +626,13 @@ END(interrupt)
 /* 0(%rsp): ~(interrupt number) */
        .macro interrupt func
        cld
+       /*
+        * Since nothing in interrupt handling code touches r12...r15 members
+        * of "struct pt_regs", and since interrupts can nest, we can save
+        * four stack slots and simultaneously provide
+        * an unwind-friendly stack layout by saving "truncated" pt_regs
+        * exactly up to rbp slot, without these members.
+        */
        ALLOC_PT_GPREGS_ON_STACK -RBP
        SAVE_C_REGS -RBP
        /* this goes to 0(%rsp) for unwinder, not for saving the value: */
@@ -641,6 +645,7 @@ END(interrupt)
        SWAPGS
 1:
        /*
+        * Save previous stack pointer, optionally switch to interrupt stack.
         * irq_count is used to check if a CPU is already on an interrupt stack
         * or not. While this is essentially redundant with preempt_count it is
         * a little cheaper to use a separate counter in the PDA (short of
@@ -681,6 +686,7 @@ ret_from_intr:
        /* Restore saved previous stack */
        popq %rsi
        CFI_DEF_CFA rsi,SS+8-RBP        /* reg/off reset after def_cfa_expr */
+       /* return code expects complete pt_regs - adjust rsp accordingly: */
        leaq ARGOFFSET-RBP(%rsi), %rsp
        CFI_DEF_CFA_REGISTER    rsp
        CFI_ADJUST_CFA_OFFSET   RBP-ARGOFFSET
@@ -692,7 +698,7 @@ exit_intr:
 
        /* Interrupt came from user space */
        /*
-        * Has a correct top of stack, but a partial stack frame
+        * Has a correct top of stack.
         * %rcx: thread info. Interrupts off.
         */
 retint_with_reschedule: