[PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path

Denys Vlasenko
2015-03-25T17:18:49Z

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: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
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 40f7760..11b7339 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -268,8 +268,11 @@ 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)
@@ -283,10 +286,7 @@ 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
@@ -297,6 +297,7 @@ 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

@@ -345,8 +346,8 @@ tracesys_phase2:
*/
GLOBAL(int_ret_from_sys_call)
DISABLE_INTERRUPTS(CLBR_NONE)
- TRACE_IRQS_OFF
int_ret_from_sys_call_irqs_off:
+ TRACE_IRQS_OFF
movl $_TIF_ALLWORK_MASK,%edi
/* edi: mask to check */
GLOBAL(int_with_check)
--
1.8.1.4