linux-kernel - Re: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer frequency from the hypervisor

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Message-ID: <20130905141636.GA22751@redhat.com>
Date:	Thu, 5 Sep 2013 17:16:36 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	KY Srinivasan <kys@...rosoft.com>
Cc:	"x86@...nel.org" <x86@...nel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"JBeulich@...e.com" <JBeulich@...e.com>,
	"bp@...en8.de" <bp@...en8.de>
Subject: Re: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer frequency
 from the hypervisor

On Wed, Sep 04, 2013 at 03:16:30PM +0000, KY Srinivasan wrote:
> 
> 
> > -----Original Message-----
> > From: Gleb Natapov [mailto:gleb@...hat.com]
> > Sent: Wednesday, September 04, 2013 2:40 AM
> > To: KY Srinivasan
> > Cc: x86@...nel.org; gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> > devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> > jasowang@...hat.com; tglx@...utronix.de; hpa@...or.com;
> > JBeulich@...e.com; bp@...en8.de
> > Subject: Re: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer frequency
> > from the hypervisor
> > 
> > On Tue, Sep 03, 2013 at 11:30:23AM -0700, K. Y. Srinivasan wrote:
> > > Hyper-V supports a mechanism for retrieving the local APIC frequency.Use this
> > and bypass
> > > the calibration code in the kernel. This would allow us to boot the Linux kernel
> > as a
> > > "modern VM" on Hyper-V where many of the legacy devices (such as PIT) are
> > not emulated.
> > >
> > > I would like to thank Olaf Hering <olaf@...fle.de>, Jan Beulich
> > <JBeulich@...e.com> and
> > > H. Peter Anvin <h.peter.anvin@...el.com> for their help in this effort.
> > >
> > > In this version of the patch, I have addressed Jan's comments.
> > >
> > > Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> > > ---
> > >  arch/x86/include/uapi/asm/hyperv.h |   19 +++++++++++++++++++
> > >  arch/x86/kernel/cpu/mshyperv.c     |   24 ++++++++++++++++++++++++
> > >  2 files changed, 43 insertions(+), 0 deletions(-)
> > >
> > > diff --git a/arch/x86/include/uapi/asm/hyperv.h
> > b/arch/x86/include/uapi/asm/hyperv.h
> > > index b80420b..b8f1c01 100644
> > > --- a/arch/x86/include/uapi/asm/hyperv.h
> > > +++ b/arch/x86/include/uapi/asm/hyperv.h
> > > @@ -27,6 +27,19 @@
> > >  #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)
> > > +
> > > +/*
> > > + * There is a single feature flag that signifies the presence of the MSR
> > > + * that can be used to retrieve both the local APIC Timer frequency as
> > > + * well as the TSC frequency.
> > > + */
> > > +
> > > +/* 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)
> > > +
> > >  /*
> > >   * Basic SynIC MSRs (HV_X64_MSR_SCONTROL through HV_X64_MSR_EOM
> > >   * and HV_X64_MSR_SINT0 through HV_X64_MSR_SINT15) available
> > > @@ -136,6 +149,12 @@
> > >  /* MSR used to read the per-partition time reference counter */
> > >  #define HV_X64_MSR_TIME_REF_COUNT		0x40000020
> > >
> > > +/* MSR used to retrieve the TSC frequency */
> > > +#define HV_X64_MSR_TSC_FREQUENCY		0x40000022
> > > +
> > You do not use this MSR in the patch, but in general how it suppose to
> > work during migration if host TSC frequency changes?
> 
> TSC related migration issues are distinct from how we calibrate the TSC frequency. This MSR
> allows you to retrieve the TSC frequency without having to do explicit calibration.
> 
But the value retrieved can be obsolete by the time guest sues it due to
migration.

> To address the migration issues; the hypervisor provides additional information on the host
> that you are running that can be used to compensate for differences in TSC across hosts.  
> 
Are you referring to "Reference TSC Page" here?

--
			Gleb.
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