Re: [RFC][PATCH 7/9] x86/tsc: Provide sched_clock_noinstr()

Peter Zijlstra
2023-05-08T21:45:30Z

On Mon, May 08, 2023 at 11:19:58PM +0200, Peter Zijlstra wrote:

> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -408,9 +408,9 @@ static u64 notrace read_hv_clock_tsc_cs(
> return read_hv_clock_tsc();
> }
>
> -static u64 notrace read_hv_sched_clock_tsc(void)
> +static u64 noinstr read_hv_sched_clock_tsc(void)
> {
> - return (read_hv_clock_tsc() - hv_sched_clock_offset) *
> + return (hv_read_tsc_page(hv_get_tsc_page()) - hv_sched_clock_offset) *
> (NSEC_PER_SEC / HV_CLOCK_HZ);
> }
>
> --- a/include/clocksource/hyperv_timer.h
> +++ b/include/clocksource/hyperv_timer.h
> @@ -38,7 +38,7 @@ extern void hv_remap_tsc_clocksource(voi
> extern unsigned long hv_get_tsc_pfn(void);
> extern struct ms_hyperv_tsc_page *hv_get_tsc_page(void);
>
> -static inline notrace u64
> +static __always_inline notrace u64
> hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg, u64 *cur_tsc)
> {
> u64 scale, offset;
> @@ -85,7 +85,7 @@ hv_read_tsc_page_tsc(const struct ms_hyp
> return mul_u64_u64_shr(*cur_tsc, scale, 64) + offset;
> }
>
> -static inline notrace u64
> +static __always_inline notrace u64
> hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)
> {
> u64 cur_tsc;

Hyper-V folks!

While reviewing all this I found the following 'gem':

hv_init_clocksource()
hv_setup_sched_clock()
paravirt_set_sched_clock(read_hv_sched_clock_msr)

read_hv_sched_clock_msr() [notrace]
read_hv_clock_msr() [notrace]
hv_get_register() *traced*
hv_get_non_nested_register() ...
hv_ghcb_msr_read()
WARN_ON(in_nmi())
...
local_irq_save()

Note that:

a) sched_clock() is used in NMI context a *LOT*
b) sched_clock() is notrace (or even noinstr with these patches)
and local_irq_save() implies tracing

Can you pretty please:

1) delete all this; or,
2) fix it in a hurry?

Thanks!