Re: [PATCH v2 04/13] arm64/arch_timer: Provide noinstr sched_clock_read() functions

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 24, 2023 at 05:40:47PM +0100, Valentin Schneider wrote:
> On 19/05/23 12:21, Peter Zijlstra wrote:
> > With the intent to provide local_clock_noinstr(), a variant of
> > local_clock() that's safe to be called from noinstr code (with the
> > assumption that any such code will already be non-preemptible),
> > prepare for things by providing a noinstr sched_clock_read() function.
> >
> > Specifically, preempt_enable_*() calls out to schedule(), which upsets
> > noinstr validation efforts.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> >  arch/arm64/include/asm/arch_timer.h  |    8 ----
> >  drivers/clocksource/arm_arch_timer.c |   60 ++++++++++++++++++++++++++---------
> >  2 files changed, 47 insertions(+), 21 deletions(-)
> >
> > --- a/arch/arm64/include/asm/arch_timer.h
> > +++ b/arch/arm64/include/asm/arch_timer.h
> > @@ -88,13 +88,7 @@ static inline notrace u64 arch_timer_rea
> >
> >  #define arch_timer_reg_read_stable(reg)					\
> >       ({								\
> > -		u64 _val;						\
> > -									\
> > -		preempt_disable_notrace();				\
> > -		_val = erratum_handler(read_ ## reg)();			\
> > -		preempt_enable_notrace();				\
> > -									\
> > -		_val;							\
> > +		erratum_handler(read_ ## reg)();			\
> >       })
> >
> >  /*
> > --- a/drivers/clocksource/arm_arch_timer.c
> > +++ b/drivers/clocksource/arm_arch_timer.c
> > @@ -191,22 +191,40 @@ u32 arch_timer_reg_read(int access, enum
> >       return val;
> >  }
> >
> > -static notrace u64 arch_counter_get_cntpct_stable(void)
> > +static noinstr u64 _arch_counter_get_cntpct_stable(void)
> >  {
> >       return __arch_counter_get_cntpct_stable();
> >  }
> >
> 
> I tripped over the underscores when reviewing this :( This must be
> getting close to the symbol length limit, but could we give this a helpful
> prefix or suffix? raw_* or *_noinstr?
> 
> > @@ -1074,6 +1092,13 @@ struct arch_timer_kvm_info *arch_timer_g
> >
> >  static void __init arch_counter_register(unsigned type)
> >  {
> > +	/*
> > +	 * Default to cp15 based access because arm64 uses this function for
> > +	 * sched_clock() before DT is probed and the cp15 method is guaranteed
> > +	 * to exist on arm64. arm doesn't use this before DT is probed so even
> > +	 * if we don't have the cp15 accessors we won't have a problem.
> > +	 */
> > +	u64 (*scr)(void) = arch_counter_get_cntvct;
> 
> So this bit sent me on a little spelunking session :-)
> 
> From a control flow perspective the initialization isn't required, but then
> I looked into the comment and found it comes from the
> arch_timer_read_counter() definition... Which itself doesn't get used by
> sched_clock() until the sched_clock_register() below!
> 
> So AFAICT that comment was true as of
> 
>   220069945b29 ("clocksource: arch_timer: Add support for memory mapped timers")
> 
> but not after a commit that came 2 months later:
> 
>   65cd4f6c99c1 ("arch_timer: Move to generic sched_clock framework")
> 
> which IIUC made arm/arm64 follow the default approach of using the
> jiffy-based sched_clock() before probing DT/ACPI and registering a "proper"
> sched_clock.
> 
> All of that to say: the comment about arch_timer_read_counter() vs early
> sched_clock() doesn't apply anymore, but I think we need to keep its
> initalization around for stuff like get_cycles(). This initialization here
> should be OK to put to the bin, though.

Something like the below folded in then?

---
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -191,7 +191,7 @@ u32 arch_timer_reg_read(int access, enum
 	return val;
 }
 
-static noinstr u64 _arch_counter_get_cntpct_stable(void)
+static noinstr u64 raw_counter_get_cntpct_stable(void)
 {
 	return __arch_counter_get_cntpct_stable();
 }
@@ -210,7 +210,7 @@ static noinstr u64 arch_counter_get_cntp
 	return __arch_counter_get_cntpct();
 }
 
-static noinstr u64 _arch_counter_get_cntvct_stable(void)
+static noinstr u64 raw_counter_get_cntvct_stable(void)
 {
 	return __arch_counter_get_cntvct_stable();
 }
@@ -1092,13 +1092,7 @@ struct arch_timer_kvm_info *arch_timer_g
 
 static void __init arch_counter_register(unsigned type)
 {
-	/*
-	 * Default to cp15 based access because arm64 uses this function for
-	 * sched_clock() before DT is probed and the cp15 method is guaranteed
-	 * to exist on arm64. arm doesn't use this before DT is probed so even
-	 * if we don't have the cp15 accessors we won't have a problem.
-	 */
-	u64 (*scr)(void) = arch_counter_get_cntvct;
+	u64 (*scr)(void);
 	u64 start_count;
 	int width;
 
@@ -1110,7 +1104,7 @@ static void __init arch_counter_register
 		    arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
 			if (arch_timer_counter_has_wa()) {
 				rd = arch_counter_get_cntvct_stable;
-				scr = _arch_counter_get_cntvct_stable;
+				scr = raw_counter_get_cntvct_stable;
 			} else {
 				rd = arch_counter_get_cntvct;
 				scr = arch_counter_get_cntvct;
@@ -1118,7 +1112,7 @@ static void __init arch_counter_register
 		} else {
 			if (arch_timer_counter_has_wa()) {
 				rd = arch_counter_get_cntpct_stable;
-				scr = _arch_counter_get_cntpct_stable;
+				scr = raw_counter_get_cntpct_stable;
 			} else {
 				rd = arch_counter_get_cntpct;
 				scr = arch_counter_get_cntpct;