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@xxxxxxxxxxxxx>
> ---
> 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.
