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

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Message-ID: <e748dc167d444e85b16c649cb8437165@SN2PR03MB061.namprd03.prod.outlook.com>
Date:	Tue, 27 Aug 2013 16:37:32 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: RE: [PATCH 1/1] X86: Hyper-V: Get the local APIC timer frequency
 from the hypervisor

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@...e.com]
> Sent: Tuesday, August 27, 2013 12:26 AM
> To: KY Srinivasan
> 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

Thanks Jan. I will fix the issues you have pointed out and resubmit the patch.

Regards,

K. Y
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