RE: [PATCH v2 1/2] clocksource/drivers/hyperv: add data structure for reference TSC MSR

[Michael Kelley (LINUX)]

From: Michael Kelley (LINUX) <mikelley@xxxxxxxxxxxxx> Sent: Thursday, October 27, 2022 6:43 AM
> From: Anirudh Rayabharam <anrayabh@xxxxxxxxxxxxxxxxxxx> Sent: Thursday,
> October 27, 2022 2:57 AM
> >
> > Add a data structure to represent the reference TSC MSR similar to
> > other MSRs. This simplifies the code for updating the MSR.
> >
> > Signed-off-by: Anirudh Rayabharam <anrayabh@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/clocksource/hyperv_timer.c | 28 ++++++++++++++--------------
> >  include/asm-generic/hyperv-tlfs.h  |  9 +++++++++
> >  2 files changed, 23 insertions(+), 14 deletions(-)
> >
> > diff --git a/drivers/clocksource/hyperv_timer.c
> b/drivers/clocksource/hyperv_timer.c
> > index bb47610bbd1c..11332c82d1af 100644
> > --- a/drivers/clocksource/hyperv_timer.c
> > +++ b/drivers/clocksource/hyperv_timer.c
> > @@ -395,25 +395,25 @@ static u64 notrace read_hv_sched_clock_tsc(void)
> >
> >  static void suspend_hv_clock_tsc(struct clocksource *arg)
> >  {
> > -	u64 tsc_msr;
> > +	union hv_reference_tsc_msr tsc_msr;
> >
> >  	/* Disable the TSC page */
> > -	tsc_msr = hv_get_register(HV_REGISTER_REFERENCE_TSC);
> > -	tsc_msr &= ~BIT_ULL(0);
> > -	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr);
> > +	tsc_msr.as_uint64 = hv_get_register(HV_REGISTER_REFERENCE_TSC);
> > +	tsc_msr.enable = 0;
> > +	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr.as_uint64);
> >  }
> >
> >
> >  static void resume_hv_clock_tsc(struct clocksource *arg)
> >  {
> >  	phys_addr_t phys_addr = virt_to_phys(&tsc_pg);
> > -	u64 tsc_msr;
> > +	union hv_reference_tsc_msr tsc_msr;
> >
> >  	/* Re-enable the TSC page */
> > -	tsc_msr = hv_get_register(HV_REGISTER_REFERENCE_TSC);
> > -	tsc_msr &= GENMASK_ULL(11, 0);
> > -	tsc_msr |= BIT_ULL(0) | (u64)phys_addr;
> > -	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr);
> > +	tsc_msr.as_uint64 = hv_get_register(HV_REGISTER_REFERENCE_TSC);
> > +	tsc_msr.enable = 1;
> > +	tsc_msr.pfn = __phys_to_pfn(phys_addr);

My previous review missed a problem here (and in the similar line below).
__phys_to_pfn() will return a PFN based on the guest page size, which might
be different from Hyper-V's page size that is always 4K.  This needs to be a
Hyper-V PFN, and we have virt_to_hvpfn() available to do just that, assuming
that function is safe to use here and in the case below. 

Michael

> > +	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr.as_uint64);
> >  }
> >
> >  #ifdef HAVE_VDSO_CLOCKMODE_HVCLOCK
> > @@ -495,7 +495,7 @@ static __always_inline void hv_setup_sched_clock(void
> > *sched_clock) {}
> >
> >  static bool __init hv_init_tsc_clocksource(void)
> >  {
> > -	u64		tsc_msr;
> > +	union hv_reference_tsc_msr tsc_msr;
> >  	phys_addr_t	phys_addr;
> >
> >  	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
> > @@ -530,10 +530,10 @@ static bool __init hv_init_tsc_clocksource(void)
> >  	 * (which already has at least the low 12 bits set to zero since
> >  	 * it is page aligned). Also set the "enable" bit, which is bit 0.
> >  	 */
> > -	tsc_msr = hv_get_register(HV_REGISTER_REFERENCE_TSC);
> > -	tsc_msr &= GENMASK_ULL(11, 0);
> > -	tsc_msr = tsc_msr | 0x1 | (u64)phys_addr;
> > -	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr);
> > +	tsc_msr.as_uint64 = hv_get_register(HV_REGISTER_REFERENCE_TSC);
> > +	tsc_msr.enable = 1;
> > +	tsc_msr.pfn = __phys_to_pfn(phys_addr);
> > +	hv_set_register(HV_REGISTER_REFERENCE_TSC, tsc_msr.as_uint64);
> >
> >  	clocksource_register_hz(&hyperv_cs_tsc, NSEC_PER_SEC/100);
> >
> > diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
> > index fdce7a4cfc6f..b17c6eeb9afa 100644
> > --- a/include/asm-generic/hyperv-tlfs.h
> > +++ b/include/asm-generic/hyperv-tlfs.h
> > @@ -102,6 +102,15 @@ struct ms_hyperv_tsc_page {
> >  	volatile s64 tsc_offset;
> >  } __packed;
> >
> > +union hv_reference_tsc_msr {
> > +	u64 as_uint64;
> > +	struct {
> > +		u64 enable:1;
> > +		u64 reserved:11;
> > +		u64 pfn:52;
> > +	} __packed;
> > +};
> > +
> >  /*
> >   * The guest OS needs to register the guest ID with the hypervisor.
> >   * The guest ID is a 64 bit entity and the structure of this ID is
> > --
> > 2.34.1
> 
> Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>