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

From: tip-bot for Denys Vlasenko
Date: Thu Apr 02 2015 - 08:25:56 EST

Commit-ID: 4416c5a6dacdddd55378e7011f9c8720d2a7470f
Gitweb: http://git.kernel.org/tip/4416c5a6dacdddd55378e7011f9c8720d2a7470f
Author: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
AuthorDate: Tue, 31 Mar 2015 19:00:03 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 1 Apr 2015 13:17:38 +0200

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

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@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1427821211-25099-1-git-send-email-dvlasenk@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
arch/x86/kernel/entry_64.S | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 6f251a5..f6e37de 100644
--- 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.
+ /*
+ * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
+ * it is too small to ever cause noticeable irq latency.
+ */

* 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 */

- /*
- * sysretq will re-enable interrupts:
- */
movq RIP(%rsp),%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.

@@ -346,8 +347,8 @@ tracesys_phase2:
+int_ret_from_sys_call_irqs_off: /* jumps come here from the irqs-off SYSRET path */
movl $_TIF_ALLWORK_MASK,%edi
/* edi: mask to check */
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