Messages in this thread | | | Date | Tue, 27 Aug 2013 08:25:38 +0100 | From | "Jan Beulich" <> | Subject | Re: [PATCH 1/1] X86: Hyper-V: Get the local APIC timer frequency from the hypervisor |
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>>> On 27.08.13 at 01:42, "K. Y. Srinivasan" <kys@microsoft.com> wrote: > Hyper-V supports a mechanism for retrieving the local API frequency.
"APIC"?
> @@ -27,6 +27,13 @@ > #define HV_X64_MSR_VP_RUNTIME_AVAILABLE (1 << 0) > /* Partition Reference Counter (HV_X64_MSR_TIME_REF_COUNT) available*/ > #define HV_X64_MSR_TIME_REF_COUNT_AVAILABLE (1 << 1) > + > +/* Local APIC timer frequency MSR (HV_X64_MSR_APIC_FREQUENCY) is available */ > +#define HV_X64_MSR_APIC_FREQUENCY_AVAILABLE (1 << 11) > + > +/* TSC frequency MSR (HV_X64_MSR_TSC_FREQUENCY) is available */ > +#define HV_X64_MSR_TSC_FREQUENCY_AVAILABLE (1 << 11)
Are these two really the same bit? If so, why two different names?
> @@ -136,6 +143,12 @@ > /* MSR used to read the per-partition time reference counter */ > #define HV_X64_MSR_TIME_REF_COUNT 0x40000020 > > +/* MSR used to retrive the TSC frequency */ > +#define HV_X64_MSR_TSC_FREQUENCY 0x40000022 > + > +/* MSR used to retrive the local APIC timer frequency */ > +#define HV_X64_MSR_APIC_FREQUENCY 0x40000023
"retrieve" (twice)?
> @@ -76,6 +77,22 @@ static void __init ms_hyperv_init_platform(void) > printk(KERN_INFO "HyperV: features 0x%x, hints 0x%x\n", > ms_hyperv.features, ms_hyperv.hints); > > + if (ms_hyperv.features & HV_X64_MSR_APIC_FREQUENCY_AVAILABLE) { > + /* > + * There is no need to calibrate APIC timer frequency; > + * nor is there a need to calibrate timer. > + */ > + legacy_pic = &null_legacy_pic;
This clearly disables more than just the calibration, so the comment may be misleading to future readers.
> + > + /* > + * Get the APIC frequency. > + */ > + rdmsrl(HV_X64_MSR_APIC_FREQUENCY, lapic_timer_frequency); > + lapic_timer_frequency /= HZ;
Is this safe? I.e. are the high 32 bits of the MSR guaranteed to be zero, now and forever?
> + printk(KERN_INFO "HyperV: LAPIC Timer Frequency: 0x%x\n", > + lapic_timer_frequency);
As a minor note, I generally recommend using %#x as being one byte shorter than the spelled out 0x%x.
Jan
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