RE: [PATCH v10 3/7] arm64: hyperv: Add Hyper-V clocksource/clockevent support

[Michael Kelley]

From: Mark Rutland <mark.rutland@xxxxxxx> Sent: Friday, May 14, 2021 5:37 AM
> 
> Hi Michael,
> 
> On Wed, May 12, 2021 at 10:37:43AM -0700, Michael Kelley wrote:
> > Add architecture specific definitions and functions needed
> > by the architecture independent Hyper-V clocksource driver.
> > Update the Hyper-V clocksource driver to be initialized
> > on ARM64.
> 
> Previously we've said that for a clocksource we must use the architected
> counter, since that's necessary for things like the VDSO to work
> correctly and efficiently.
> 
> Given that, I'm a bit confused that we're registering a per-cpu
> clocksource that is in part based on the architected counter. Likewise,
> I don't entirely follow why it's necessary to PV the clock_event_device.
> 
> Are the architected counter and timer reliable without this PV
> infrastructure? Why do we need to PV either of those?
> 
> Thanks,
> Mark.
> 

For the clocksource, we have a requirement to live migrate VMs
between Hyper-V hosts running on hardware that may have different
arch counter frequencies (it's not conformant to the ARM v8.6 1 GHz
requirement).  The Hyper-V virtualization does scaling to handle the
frequency difference.  And yes, there's a tradeoff with vDSO not
working, though we have an out-of-tree vDSO implementation that
we can use when necessary.

For clockevents, the only timer interrupt that Hyper-V provides
in a guest VM is its virtualized "STIMER" interrupt.  There's no
virtualization of the ARM arch timer in the guest.

Michael

> >
> > Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> > Reviewed-by: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/mshyperv.h  | 12 ++++++++++++
> >  drivers/clocksource/hyperv_timer.c | 14 ++++++++++++++
> >  2 files changed, 26 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/mshyperv.h b/arch/arm64/include/asm/mshyperv.h
> > index c448704..b17299c 100644
> > --- a/arch/arm64/include/asm/mshyperv.h
> > +++ b/arch/arm64/include/asm/mshyperv.h
> > @@ -21,6 +21,7 @@
> >  #include <linux/types.h>
> >  #include <linux/arm-smccc.h>
> >  #include <asm/hyperv-tlfs.h>
> > +#include <clocksource/arm_arch_timer.h>
> >
> >  /*
> >   * Declare calls to get and set Hyper-V VP register values on ARM64, which
> > @@ -41,6 +42,17 @@ static inline u64 hv_get_register(unsigned int reg)
> >  	return hv_get_vpreg(reg);
> >  }
> >
> > +/* Define the interrupt ID used by STIMER0 Direct Mode interrupts. This
> > + * value can't come from ACPI tables because it is needed before the
> > + * Linux ACPI subsystem is initialized.
> > + */
> > +#define HYPERV_STIMER0_VECTOR	31
> > +
> > +static inline u64 hv_get_raw_timer(void)
> > +{
> > +	return arch_timer_read_counter();
> > +}
> > +
> >  /* SMCCC hypercall parameters */
> >  #define HV_SMCCC_FUNC_NUMBER	1
> >  #define HV_FUNC_ID	ARM_SMCCC_CALL_VAL(			\
> > diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> > index 977fd05..270ad9c 100644
> > --- a/drivers/clocksource/hyperv_timer.c
> > +++ b/drivers/clocksource/hyperv_timer.c
> > @@ -569,3 +569,17 @@ void __init hv_init_clocksource(void)
> >  	hv_setup_sched_clock(read_hv_sched_clock_msr);
> >  }
> >  EXPORT_SYMBOL_GPL(hv_init_clocksource);
> > +
> > +/* Initialize everything on ARM64 */
> > +static int __init hyperv_timer_init(struct acpi_table_header *table)
> > +{
> > +	if (!hv_is_hyperv_initialized())
> > +		return -EINVAL;
> > +
> > +	hv_init_clocksource();
> > +	if (hv_stimer_alloc(true))
> > +		return -EINVAL;
> > +
> > +	return 0;
> > +}
> > +TIMER_ACPI_DECLARE(hyperv, ACPI_SIG_GTDT, hyperv_timer_init);
> > --
> > 1.8.3.1
> >