[v5,2/2] arm64: enable context tracking

Message ID 1401130573-7443-3-git-send-email-larry.bassel@linaro.org
State New
Headers show

Commit Message

Larry Bassel May 26, 2014, 6:56 p.m.
Make calls to ct_user_enter when the kernel is exited
and ct_user_exit when the kernel is entered (in el0_da,
el0_ia, el0_svc, el0_irq and all of the "error" paths).

These macros expand to function calls which will only work
properly if el0_sync and related code has been rearranged
(in a previous patch of this series).

The calls to ct_user_exit are made after hw debugging has been
enabled (enable_dbg_and_irq).

The call to ct_user_enter is made at the beginning of the
kernel_exit macro.

This patch is based on earlier work by Kevin Hilman.
Save/restore optimizations were also done by Kevin.

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Larry Bassel <larry.bassel@linaro.org>
---
 arch/arm64/Kconfig                   |  1 +
 arch/arm64/include/asm/thread_info.h |  1 +
 arch/arm64/kernel/entry.S            | 46 ++++++++++++++++++++++++++++++++++++
 3 files changed, 48 insertions(+)

Comments

Will Deacon May 28, 2014, 11:44 a.m. | #1
Hi Larry,

On Mon, May 26, 2014 at 07:56:13PM +0100, Larry Bassel wrote:
> Make calls to ct_user_enter when the kernel is exited
> and ct_user_exit when the kernel is entered (in el0_da,
> el0_ia, el0_svc, el0_irq and all of the "error" paths).
> 
> These macros expand to function calls which will only work
> properly if el0_sync and related code has been rearranged
> (in a previous patch of this series).
> 
> The calls to ct_user_exit are made after hw debugging has been
> enabled (enable_dbg_and_irq).
> 
> The call to ct_user_enter is made at the beginning of the
> kernel_exit macro.
> 
> This patch is based on earlier work by Kevin Hilman.
> Save/restore optimizations were also done by Kevin.

Apologies if we've discussed this before (it rings a bell), but why are we
penalising the fast syscall path with this? Shouldn't TIF_NOHZ contribute to
out _TIF_WORK_MASK, then we could do the tracking on the syscall slow path?

I think that would tidy up your mov into x19 too.

Also -- how do you track ret_from_fork in the child with these patches?

Will
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Kevin Hilman May 28, 2014, 3:55 p.m. | #2
Hi Will,

Will Deacon <will.deacon@arm.com> writes:

> On Mon, May 26, 2014 at 07:56:13PM +0100, Larry Bassel wrote:
>> Make calls to ct_user_enter when the kernel is exited
>> and ct_user_exit when the kernel is entered (in el0_da,
>> el0_ia, el0_svc, el0_irq and all of the "error" paths).
>> 
>> These macros expand to function calls which will only work
>> properly if el0_sync and related code has been rearranged
>> (in a previous patch of this series).
>> 
>> The calls to ct_user_exit are made after hw debugging has been
>> enabled (enable_dbg_and_irq).
>> 
>> The call to ct_user_enter is made at the beginning of the
>> kernel_exit macro.
>> 
>> This patch is based on earlier work by Kevin Hilman.
>> Save/restore optimizations were also done by Kevin.
>
> Apologies if we've discussed this before (it rings a bell), but why are we
> penalising the fast syscall path with this? Shouldn't TIF_NOHZ contribute to
> out _TIF_WORK_MASK, then we could do the tracking on the syscall slow path?

I'll answer here since Larry inherited this design decision from me.

I considered (and even implemented) forcing the slow syscall path
based on TIF_NOHZ but decided (perhaps wrongly) not to.  I guess the
choice is between:

- forcing the overhead of syscall tracing path on all
  TIF_NOHZ processes

- forcing the (much smaller) ct_user_exit overhead on all syscalls,
  (including the fast syscall path)

I had decided that the former was better, but as I write this, I'm
thinking that the NOHZ tasks should probably eat the extra overhead
since we expect their interactions with the kernel to be minimal anyways
(part of the goal of full NOHZ.)

Ultimately, I'm OK with either way and have the other version ready.

> I think that would tidy up your mov into x19 too.

That's correct.  If we force the syscall_trace path, the ct_user_enter
wouldn't have to do any context save/restore.

> Also -- how do you track ret_from_fork in the child with these patches?

Not sure I follow the question, but ret_from_fork calls
ret_to_user, which calls kernel_exit, which calls ct_user_enter.

Kevin
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Will Deacon May 28, 2014, 6:49 p.m. | #3
On Wed, May 28, 2014 at 04:55:39PM +0100, Kevin Hilman wrote:
> Hi Will,

Hey Kevin,

> Will Deacon <will.deacon@arm.com> writes:
> > Apologies if we've discussed this before (it rings a bell), but why are we
> > penalising the fast syscall path with this? Shouldn't TIF_NOHZ contribute to
> > out _TIF_WORK_MASK, then we could do the tracking on the syscall slow path?
> 
> I'll answer here since Larry inherited this design decision from me.
> 
> I considered (and even implemented) forcing the slow syscall path
> based on TIF_NOHZ but decided (perhaps wrongly) not to.  I guess the
> choice is between:
> 
> - forcing the overhead of syscall tracing path on all
>   TIF_NOHZ processes
> 
> - forcing the (much smaller) ct_user_exit overhead on all syscalls,
>   (including the fast syscall path)
> 
> I had decided that the former was better, but as I write this, I'm
> thinking that the NOHZ tasks should probably eat the extra overhead
> since we expect their interactions with the kernel to be minimal anyways
> (part of the goal of full NOHZ.)
> 
> Ultimately, I'm OK with either way and have the other version ready.

I was just going by the comment in kernel/context_tracking.c:

 * The context tracking uses the syscall slow path to implement its user-kernel
 * boundaries probes on syscalls. This way it doesn't impact the syscall fast
 * path on CPUs that don't do context tracking.

which doesn't match what the current patch does. It also makes it sounds
like context tracking is really a per-CPU thing, but I've never knowingly
used it before.

I think putting this on the slowpath is inline with the expectations in the
core code.

> > I think that would tidy up your mov into x19 too.
> 
> That's correct.  If we force the syscall_trace path, the ct_user_enter
> wouldn't have to do any context save/restore.

That would be nice.

> > Also -- how do you track ret_from_fork in the child with these patches?
> 
> Not sure I follow the question, but ret_from_fork calls
> ret_to_user, which calls kernel_exit, which calls ct_user_enter.

Sorry, I got myself in a muddle. I noticed that x19 is live in ret_from_fork
so made a mental note to check that is ok (I think it is) but then concluded
incorrectly that you don't trace there.

Will
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Patch

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index e759af5..ef18ae5 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -55,6 +55,7 @@  config ARM64
 	select RTC_LIB
 	select SPARSE_IRQ
 	select SYSCTL_EXCEPTION_TRACE
+	select HAVE_CONTEXT_TRACKING
 	help
 	  ARM 64-bit (AArch64) Linux support.
 
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 720e70b..301ea6a 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -108,6 +108,7 @@  static inline struct thread_info *current_thread_info(void)
 #define TIF_SINGLESTEP		21
 #define TIF_32BIT		22	/* 32bit process */
 #define TIF_SWITCH_MM		23	/* deferred switch_mm */
+#define TIF_NOHZ                24
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index c6bc1a3..0605963 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -30,6 +30,42 @@ 
 #include <asm/unistd32.h>
 
 /*
+ * Context tracking subsystem.  Used to instrument transitions
+ * between user and kernel mode.
+ */
+	.macro ct_user_exit, restore = 0
+#ifdef CONFIG_CONTEXT_TRACKING
+	bl	context_tracking_user_exit
+	.if \restore == 1
+	/*
+	 * Save/restore needed during syscalls.  Restore syscall arguments from
+	 * the values already saved on stack during kernel_entry.
+	 */
+	ldp	x0, x1, [sp]
+	ldp	x2, x3, [sp, #S_X2]
+	ldp	x4, x5, [sp, #S_X4]
+	ldp	x6, x7, [sp, #S_X6]
+	.endif
+#endif
+	.endm
+
+	.macro ct_user_enter, save = 0
+#ifdef CONFIG_CONTEXT_TRACKING
+	.if \save == 1
+	/*
+	 * We only have to save/restore x0 on the fast syscall path where
+	 * x0 contains the syscall return.
+	 */
+	mov	x19, x0
+	.endif
+	bl	context_tracking_user_enter
+	.if \save == 1
+	mov	x0, x19
+	.endif
+#endif
+	.endm
+
+/*
  * Bad Abort numbers
  *-----------------
  */
@@ -91,6 +127,7 @@ 
 	.macro	kernel_exit, el, ret = 0
 	ldp	x21, x22, [sp, #S_PC]		// load ELR, SPSR
 	.if	\el == 0
+	ct_user_enter \ret
 	ldr	x23, [sp, #S_SP]		// load return stack pointer
 	.endif
 	.if	\ret
@@ -318,6 +355,7 @@  el1_irq:
 	bl	trace_hardirqs_off
 #endif
 
+	ct_user_exit
 	irq_handler
 
 #ifdef CONFIG_PREEMPT
@@ -427,6 +465,7 @@  el0_da:
 	mrs	x26, far_el1
 	// enable interrupts before calling the main handler
 	enable_dbg_and_irq
+	ct_user_exit
 	bic	x0, x26, #(0xff << 56)
 	mov	x1, x25
 	mov	x2, sp
@@ -439,6 +478,7 @@  el0_ia:
 	mrs	x26, far_el1
 	// enable interrupts before calling the main handler
 	enable_dbg_and_irq
+	ct_user_exit
 	mov	x0, x26
 	orr	x1, x25, #1 << 24		// use reserved ISS bit for instruction aborts
 	mov	x2, sp
@@ -449,6 +489,7 @@  el0_fpsimd_acc:
 	 * Floating Point or Advanced SIMD access
 	 */
 	enable_dbg
+	ct_user_exit
 	mov	x0, x25
 	mov	x1, sp
 	adr	lr, ret_to_user
@@ -458,6 +499,7 @@  el0_fpsimd_exc:
 	 * Floating Point or Advanced SIMD exception
 	 */
 	enable_dbg
+	ct_user_exit
 	mov	x0, x25
 	mov	x1, sp
 	adr	lr, ret_to_user
@@ -480,6 +522,7 @@  el0_undef:
 	 */
 	// enable interrupts before calling the main handler
 	enable_dbg_and_irq
+	ct_user_exit
 	mov	x0, sp
 	adr	lr, ret_to_user
 	b	do_undefinstr
@@ -494,10 +537,12 @@  el0_dbg:
 	mov	x2, sp
 	bl	do_debug_exception
 	enable_dbg
+	ct_user_exit
 	mov	x0, x26
 	b	ret_to_user
 el0_inv:
 	enable_dbg
+	ct_user_exit
 	mov	x0, sp
 	mov	x1, #BAD_SYNC
 	mrs	x2, esr_el1
@@ -618,6 +663,7 @@  el0_svc:
 el0_svc_naked:					// compat entry point
 	stp	x0, scno, [sp, #S_ORIG_X0]	// save the original x0 and syscall number
 	enable_dbg_and_irq
+	ct_user_exit 1
 
 	ldr	x16, [tsk, #TI_FLAGS]		// check for syscall tracing
 	tbnz	x16, #TIF_SYSCALL_TRACE, __sys_trace // are we tracing syscalls?