linux-kernel - Re: [PATCH 1/1] X86: Hyper-V: Get the local APIC timer frequency from the hypervisor

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Date:	Tue, 27 Aug 2013 08:25:38 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
Cc:	<olaf@...fle.de>, <apw@...onical.com>, <x86@...nel.org>,
	<devel@...uxdriverproject.org>, <gregkh@...uxfoundation.org>,
	<jasowang@...hat.com>, <linux-kernel@...r.kernel.org>,
	<hpa@...or.com>
Subject: Re: [PATCH 1/1] X86: Hyper-V: Get the local APIC timer
 frequency from the hypervisor

>>> On 27.08.13 at 01:42, "K. Y. Srinivasan" <kys@...rosoft.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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