RE: [PATCH] Hyper-V: fix for unwanted manipulation of sched_clock when TSC marked unstable

[Ani Sinha]

On Tue, 13 Jul 2021, Michael Kelley wrote:

> From: Ani Sinha <ani@xxxxxxxxxxx> Sent: Monday, July 12, 2021 8:05 PM
> >
> > Marking TSC as unstable has a side effect of marking sched_clock as
> > unstable when TSC is still being used as the sched_clock. This is not
> > desirable. Hyper-V ultimately uses a paravirtualized clock source that
> > provides a stable scheduler clock even on systems without TscInvariant
> > CPU capability. Hence, mark_tsc_unstable() call should be called _after_
> > scheduler clock has been changed to the paravirtualized clocksource. This
> > will prevent any unwanted manipulation of the sched_clock. Only TSC will
> > be correctly marked as unstable.
> >
> > Signed-off-by: Ani Sinha <ani@xxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/mshyperv.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> > index 22f13343b5da..715458b7729a 100644
> > --- a/arch/x86/kernel/cpu/mshyperv.c
> > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > @@ -370,8 +370,6 @@ static void __init ms_hyperv_init_platform(void)
> >  	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
> >  		wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
> >  		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> > -	} else {
> > -		mark_tsc_unstable("running on Hyper-V");
> >  	}
> >
> >  	/*
> > @@ -432,6 +430,12 @@ static void __init ms_hyperv_init_platform(void)
> >  	/* Register Hyper-V specific clocksource */
> >  	hv_init_clocksource();
> >  #endif
> > +	/* TSC should be marked as unstable only after Hyper-V
> > +	 * clocksource has been initialized. This ensures that the
> > +	 * stability of the sched_clock is not altered.
> > +	 */
>
> For multi-line comments like the above, the first comment line
> should just be "/*".  So:

Hmm, checkpatch.pl in kernel tree did not complain :

total: 0 errors, 0 warnings, 20 lines checked

0001-Hyper-V-fix-for-unwanted-manipulation-of-sched_clock.patch has no
obvious style problems and is ready for submission.

However, I do know from my experience of submitting Qemu patches last
year that this is a requirement imposed by the Qemu community as
checkpatch.pl in qemu tree would complain otherwise. I also took a peek at
the Qemu git history. It seems they imported this check from the kernel's
checkpatch.pl with this commit in Qemu tree:

commit 8c06fbdf36bf4d4d486116200248730887a4d7d6
Author: Peter Maydell <peter.maydell@xxxxxxxxxx>
Date:   Fri Dec 14 13:30:48 2018 +0000

    scripts/checkpatch.pl: Enforce multiline comment syntax

Which adds this rule:

+               # Block comments use /* on a line of its own
+               if ($rawline !~ m@^\+.*/\*.*\*/[ \t]*$@ &&      #inline /*...*/
+                   $rawline =~ m@^\+.*/\*\*?[ \t]*.+[ \t]*$@) { # /* or /** non-blank
+                       WARN("Block comments use a leading /* on a separate line\n" . $herecurr);
+               }


But in kernel there is no such rule. Hmm. strange!


>
> 	/*
> 	 * TSC should be marked as unstable only after Hyper-V
> 	 * clocksource has been initialized. This ensures that the
> 	 * stability of the sched_clock is not altered.
> 	 */
>
>
> > +	if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
> > +		mark_tsc_unstable("running on Hyper-V");
> >  }
> >
> >  static bool __init ms_hyperv_x2apic_available(void)
> > --
> > 2.25.1
>
> Modulo the comment format,
>
> Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> Tested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
>