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

[Michael Kelley (LINUX)]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Sent: Thursday, May 11, 2023 1:24 PM
> 
> On Mon, May 08, 2023 at 11:30:43PM +0000, Wei Liu wrote:
> > 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?
> 
> You do have TSC support -- which is what I fixed for you. It's just the
> whole MSR thing that is comically broken.
> 
> You could do a read_hv_clock_msr() implementation using
> __rdmsr() and add some sanity checking that anything GHCB using (SEV?)
> *will* use TSC.
> 
> Anyway, will you guys do that, or should I pull out the chainsaw and fix
> it for you?

Peter -- I'll work on a fix.  But it will be the first half of next week before
I can do it.

Michael