x86/asm/entry/64: Do not TRACE_IRQS fast SYSRET64 path

SYSRET code path has a small irq-off block.
On this code path, TRACE_IRQS_ON can't be called right before
interrupts are enabled for real, we can't clobber registers
there. So current code does it earlier, in a safe place.

But with this, TRACE_IRQS_OFF/ON frames just two fast
instructions, which is ridiculous: now most of irq-off block is
_outside_ of the framing.

Do the same thing that we do on SYSCALL entry: do not track this
irq-off block, it is very small to ever cause noticeable irq
latency.

Be careful: make sure that "jnz int_ret_from_sys_call_irqs_off"
now does invoke TRACE_IRQS_OFF - move
int_ret_from_sys_call_irqs_off label before TRACE_IRQS_OFF.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
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: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1427821211-25099-1-git-send-email-dvlasenk@redhat.com
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 6f251a5..f6e37de 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -269,8 +269,11 @@ system_call_fastpath:
  * Has incompletely filled pt_regs.
  */
        LOCKDEP_SYS_EXIT
+       /*
+        * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
+        * it is too small to ever cause noticeable irq latency.
+        */
        DISABLE_INTERRUPTS(CLBR_NONE)
-       TRACE_IRQS_OFF
 
        /*
         * We must check ti flags with interrupts (or at least preemption)
@@ -284,10 +287,7 @@ system_call_fastpath:
        jnz int_ret_from_sys_call_irqs_off      /* Go to the slow path */
 
        CFI_REMEMBER_STATE
-       /*
-        * sysretq will re-enable interrupts:
-        */
-       TRACE_IRQS_ON
+
        RESTORE_C_REGS_EXCEPT_RCX_R11
        movq    RIP(%rsp),%rcx
        CFI_REGISTER    rip,rcx
@@ -298,6 +298,7 @@ system_call_fastpath:
         * 64bit SYSRET restores rip from rcx,
         * rflags from r11 (but RF and VM bits are forced to 0),
         * cs and ss are loaded from MSRs.
+        * Restoration of rflags re-enables interrupts.
         */
        USERGS_SYSRET64
 
@@ -346,8 +347,8 @@ tracesys_phase2:
  */
 GLOBAL(int_ret_from_sys_call)
        DISABLE_INTERRUPTS(CLBR_NONE)
+int_ret_from_sys_call_irqs_off: /* jumps come here from the irqs-off SYSRET path */
        TRACE_IRQS_OFF
-int_ret_from_sys_call_irqs_off:
        movl $_TIF_ALLWORK_MASK,%edi
        /* edi: mask to check */
 GLOBAL(int_with_check)