RE: [PATCH 09/10] clocksource/drivers/hyper-v: Set clocksource rating based on Hyper-V feature

[Michael Kelley <mikelley@microsoft.com>]

From: Boqun Feng <boqun.feng@gmail.com> Sent: Monday, February 22, 2021 8:01 AM
> 
> On Wed, Jan 27, 2021 at 12:23:44PM -0800, Michael Kelley wrote:
> > On x86/x64, the TSC clocksource is available in a Hyper-V VM only if
> > Hyper-V provides the TSC_INVARIANT flag. The rating on the Hyper-V
> > Reference TSC page clocksource is currently set so that it will not
> > override the TSC clocksource in this case.  Alternatively, if the TSC
> > clocksource is not available, then the Hyper-V clocksource is used.
> >
> > But on ARM64, the Hyper-V Reference TSC page clocksource should
> > override the ARM arch counter, since the Hyper-V clocksource provides
> > scaling and offsetting during live migrations that is not provided
> > for the ARM arch counter.
> >
> > To get the needed behavior for both x86/x64 and ARM64, tweak the
> > logic by defaulting the Hyper-V Reference TSC page clocksource
> > rating to a large value that will always override.  If the Hyper-V
> > TSC_INVARIANT flag is set, then reduce the rating so that it will not
> > override the TSC.
> >
> > While the logic for getting there is slightly different, the net
> > result in the normal cases is no functional change.
> >
> 
> One question here, please see below:
> 
> > Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> > ---
> >  drivers/clocksource/hyperv_timer.c | 23 +++++++++++++----------
> >  1 file changed, 13 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> > index a2bee50..edf2d43 100644
> > --- a/drivers/clocksource/hyperv_timer.c
> > +++ b/drivers/clocksource/hyperv_timer.c
> > @@ -302,14 +302,6 @@ void hv_stimer_global_cleanup(void)
> >   * the other that uses the TSC reference page feature as defined in the
> >   * TLFS.  The MSR version is for compatibility with old versions of
> >   * Hyper-V and 32-bit x86.  The TSC reference page version is preferred.
> > - *
> > - * The Hyper-V clocksource ratings of 250 are chosen to be below the
> > - * TSC clocksource rating of 300.  In configurations where Hyper-V offers
> > - * an InvariantTSC, the TSC is not marked "unstable", so the TSC clocksource
> > - * is available and preferred.  With the higher rating, it will be the
> > - * default.  On older hardware and Hyper-V versions, the TSC is marked
> > - * "unstable", so no TSC clocksource is created and the selected Hyper-V
> > - * clocksource will be the default.
> >   */
> >
> >  u64 (*hv_read_reference_counter)(void);
> > @@ -380,7 +372,7 @@ static int hv_cs_enable(struct clocksource *cs)
> >
> >  static struct clocksource hyperv_cs_tsc = {
> >  	.name	= "hyperv_clocksource_tsc_page",
> > -	.rating	= 250,
> > +	.rating	= 500,
> >  	.read	= read_hv_clock_tsc_cs,
> >  	.mask	= CLOCKSOURCE_MASK(64),
> >  	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
> > @@ -417,7 +409,7 @@ static u64 notrace read_hv_sched_clock_msr(void)
> >
> >  static struct clocksource hyperv_cs_msr = {
> >  	.name	= "hyperv_clocksource_msr",
> > -	.rating	= 250,
> > +	.rating	= 500,
> 
> Before this patch, since the ".rating" of hyper_cs_msr is 250 which is
> smaller than the TSC clocksource rating, the TSC clocksource is better.
> After this patch, in the case where HV_MSR_REFERENCE_TSC_AVAILABLE bit
> is 0, we make hyperv_cs_msr better than the TSC clocksource (and we
> don't lower the rating of hyperv_cs_msr if TSC_INVARIANT is not
> offered), right?  Could you explain why we need the change? Or maybe I'm
> missing something?
>

You make a good point.  The code path that sets hyperv_cs_tsc.rating
to 250 should also be setting hyperv_cs_msr.rating to 250.  The reality
is that the hyperv_cs_msr clock is a backup that is never used under
normal circumstances, so I didn't pay careful attention to that case.
I'll fix it.

Michael

> 
> Regards,
> Boqun
> 
> >  	.read	= read_hv_clock_msr_cs,
> >  	.mask	= CLOCKSOURCE_MASK(64),
> >  	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
> > @@ -452,6 +444,17 @@ static bool __init hv_init_tsc_clocksource(void)
> >  	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
> >  		return false;
> >
> > +	/*
> > +	 * If Hyper-V offers TSC_INVARIANT, then the virtualized TSC correctly
> > +	 * handles frequency and offset changes due to live migration,
> > +	 * pause/resume, and other VM management operations.  So lower the
> > +	 * Hyper-V Reference TSC rating, causing the generic TSC to be used.
> > +	 * TSC_INVARIANT is not offered on ARM64, so the Hyper-V Reference
> > +	 * TSC will be preferred over the virtualized ARM64 arch counter.
> > +	 */
> > +	if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT)
> > +		hyperv_cs_tsc.rating = 250;
> > +
> >  	hv_read_reference_counter = read_hv_clock_tsc;
> >  	phys_addr = virt_to_phys(hv_get_tsc_page());
> >
> > --
> > 1.8.3.1
> >