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

[Michael Kelley]

From: Ani Sinha <ani@xxxxxxxxxxx> Sent: Tuesday, July 13, 2021 10:49 AM
> 
> 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!
> 
> 

See section 8 of "Documentation/process/coding-style.rst" in a Linux kernel
source code tree. :-)

Michael