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

[Denys Vlasenko <dvlasenk@redhat.com>]

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
---

Changes in v2: added comment

 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 9c8661c..658cf2e 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.
  */
 	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
 
@@ -347,8 +348,8 @@ 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)
-- 
1.8.1.4