Re: [patch V4 part 2 10/18] x86/entry/64: Check IF in __preempt_enable_notrace() thunk

[Alexandre Chartre]

On 5/5/20 3:41 PM, Thomas Gleixner wrote:

The preempt_enable_notrace() ASM thunk is called from tracing, entry code
RCU and other places which are already in or going to be in the noinstr
section which protects sensitve code from being instrumented.

typo: "sensitve"

alex.

Calls out of these sections happen with interrupts disabled, which is
handled in C code, but the push regs, call, pop regs sequence can be
completely avoided in this case.

This is also a preparatory step for annotating the call from the thunk to
preempt_enable_notrace() safe from a noinstr section.

Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
  arch/x86/entry/thunk_64.S       |   27 +++++++++++++++++++++++----
  arch/x86/include/asm/irqflags.h |    3 +--
  arch/x86/include/asm/paravirt.h |    3 +--
  3 files changed, 25 insertions(+), 8 deletions(-)

--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -9,10 +9,28 @@
  #include "calling.h"
  #include <asm/asm.h>
  #include <asm/export.h>
+#include <asm/irqflags.h>
+
+.code64

/* rdi: arg1 ... normal C conventions. rax is saved/restored. */

-       .macro THUNK name, func, put_ret_addr_in_rdi=0
+       .macro THUNK name, func, put_ret_addr_in_rdi=0, check_if=0
  SYM_FUNC_START_NOALIGN(\name)
+
+       .if \check_if
+       /*
+        * Check for interrupts disabled right here. No point in
+        * going all the way down
+        */
+       pushq   %rax
+       SAVE_FLAGS(CLBR_RAX)
+       testl   $X86_EFLAGS_IF, %eax
+       popq    %rax
+       jnz     1f
+       ret
+1:
+       .endif
+
        pushq %rbp
        movq %rsp, %rbp

@@ -38,14 +56,15 @@ SYM_FUNC_END(\name)

        .endm

#ifdef CONFIG_TRACE_IRQFLAGS

-       THUNK trace_hardirqs_on_thunk,trace_hardirqs_on_caller,1
-       THUNK trace_hardirqs_off_thunk,trace_hardirqs_off_caller,1
+       THUNK trace_hardirqs_on_thunk,trace_hardirqs_on_caller, 
put_ret_addr_in_rdi=1
+       THUNK trace_hardirqs_off_thunk,trace_hardirqs_off_caller, 
put_ret_addr_in_rdi=1
  #endif

#ifdef CONFIG_PREEMPTION

        THUNK preempt_schedule_thunk, preempt_schedule
-       THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
        EXPORT_SYMBOL(preempt_schedule_thunk)
+
+       THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace, 
check_if=1
        EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
  #endif

--- a/arch/x86/include/asm/irqflags.h

+++ b/arch/x86/include/asm/irqflags.h
@@ -127,9 +127,8 @@ static inline notrace unsigned long arch
  #define DISABLE_INTERRUPTS(x) cli

#ifdef CONFIG_X86_64

-#ifdef CONFIG_DEBUG_ENTRY
+
  #define SAVE_FLAGS(x)         pushfq; popq %rax
-#endif

#define SWAPGS swapgs

  /*
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -907,14 +907,13 @@ extern void default_banner(void);
                  ANNOTATE_RETPOLINE_SAFE;                              \
                  jmp PARA_INDIRECT(pv_ops+PV_CPU_usergs_sysret64);)

-#ifdef CONFIG_DEBUG_ENTRY

  #define SAVE_FLAGS(clobbers)                                        \
        PARA_SITE(PARA_PATCH(PV_IRQ_save_fl),                       \
                  PV_SAVE_REGS(clobbers | CLBR_CALLEE_SAVE);        \
                  ANNOTATE_RETPOLINE_SAFE;                          \
                  call PARA_INDIRECT(pv_ops+PV_IRQ_save_fl);        \
                  PV_RESTORE_REGS(clobbers | CLBR_CALLEE_SAVE);)
-#endif
+
  #endif /* CONFIG_PARAVIRT_XXL */
  #endif        /* CONFIG_X86_64 */