x86/asm/entry/64: Fix comments
- Misleading and slightly incorrect comments in "struct pt_regs" are
fixed (four instances).
- Fix incorrect comment atop EMPTY_FRAME macro.
- Explain in more detail what we do with stack layout during hw interrupt.
- Correct comments about "partial stack frame" which are no longer
true.
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1423778052-21038-3-git-send-email-dvlasenk@redhat.com
Link: http://lkml.kernel.org/r/e1f4429c491fe6ceeddb879dea2786e0f8920f9c.1424989793.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index e8372e0..695f4d4 100644
--- 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 @@
.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 @@
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 @@
/* 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 @@
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 @@
/* 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 @@
/* 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:
