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

[Wei Liu]

On Mon, May 08, 2023 at 11:44:19PM +0200, Peter Zijlstra wrote:
> 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
> 

Tianyu and Michael, what's your thought on this?

Is the MSR-based GHCB usable at this point?

What other clock source can be used?

Thanks,
Wei.

> 
> Can you pretty please:
> 
>  1) delete all this; or,
>  2) fix it in a hurry?
> 
> Thanks!