From: KY Srinivasan <kys@microsoft.com>
To: Gleb Natapov <gleb@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>, "x86@kernel.org" <x86@kernel.org>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
        "jasowang@redhat.com" <jasowang@redhat.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "JBeulich@suse.com" <JBeulich@suse.com>, "bp@alien8.de" <bp@alien8.de>
Subject: RE: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer frequency
 from the hypervisor
Thread-Topic: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer
 frequency from the hypervisor
Thread-Index: AQHOqM30I+mkPB2b1kKq/5YqjRysqJnC17CggAAG+ACAABQ5UIAAikAAgABK5hCABkKSgIAAYOzQ
Date: Tue, 17 Sep 2013 19:53:36 +0000
Message-ID: <2b4a9a0cff5143c59a0802a2e425c4db@SN2PR03MB061.namprd03.prod.outlook.com>
References: <1378233023-4512-1-git-send-email-kys@microsoft.com>
 <dd29077371564a14b2d317d2291db8a9@SN2PR03MB061.namprd03.prod.outlook.com>
 <52325C1E.90101@zytor.com>
 <50f92d9c16a54e5bb6fa14ef72e5fdfb@SN2PR03MB061.namprd03.prod.outlook.com>
 <20130913095526.GE18381@redhat.com>
 <7737cb643a664b6fb39021b4d364b5cf@SN2PR03MB061.namprd03.prod.outlook.com>
 <20130917135918.GY17294@redhat.com>
In-Reply-To: <20130917135918.GY17294@redhat.com>
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> -----Original Message-----
> From: Gleb Natapov [mailto:gleb@redhat.com]
> Sent: Tuesday, September 17, 2013 6:59 AM
> To: KY Srinivasan
> Cc: H. Peter Anvin; x86@kernel.org; gregkh@linuxfoundation.org; linux-
> kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.de;
> apw@canonical.com; jasowang@redhat.com; tglx@linutronix.de;
> JBeulich@suse.com; bp@alien8.de
> Subject: Re: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer frequency
> from the hypervisor
> 
> On Fri, Sep 13, 2013 at 02:28:35PM +0000, KY Srinivasan wrote:
> >
> >
> > > -----Original Message-----
> > > From: Gleb Natapov [mailto:gleb@redhat.com]
> > > Sent: Friday, September 13, 2013 2:55 AM
> > > To: KY Srinivasan
> > > Cc: H. Peter Anvin; x86@kernel.org; gregkh@linuxfoundation.org; linux-
> > > kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.de;
> > > apw@canonical.com; jasowang@redhat.com; tglx@linutronix.de;
> > > JBeulich@suse.com; bp@alien8.de
> > > Subject: Re: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer frequency
> > > from the hypervisor
> > >
> > > On Fri, Sep 13, 2013 at 01:43:09AM +0000, KY Srinivasan wrote:
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: H. Peter Anvin [mailto:hpa@zytor.com]
> > > > > Sent: Thursday, September 12, 2013 5:28 PM
> > > > > To: KY Srinivasan
> > > > > Cc: x86@kernel.org; gregkh@linuxfoundation.org; linux-
> > > kernel@vger.kernel.org;
> > > > > devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> > > > > jasowang@redhat.com; tglx@linutronix.de; JBeulich@suse.com;
> > > bp@alien8.de
> > > > > Subject: Re: [PATCH V2 1/1] X86: Hyper-V: Get the local APIC timer
> frequency
> > > > > from the hypervisor
> > > > >
> > > > > On 09/12/2013 05:06 PM, KY Srinivasan wrote:
> > > > > >
> > > > > > Peter,
> > > > > >
> > > > > > Let me know if you want me to address any additional issues in this
> patch.
> > > > > >
> > > > >
> > > > > Please address Jan and Gleb's feedback.
> > > >
> > > > Gleb's feedback was a question and I answered that as I did Jan's feedback
> as
> > > well.
> > > > Gleb, Jan, please let me know if there is something else you want
> addressed
> > > here.
> > > >
> > > No, I am just interesting to know some details about the interface since
> > > I cannot find it documented in Hyper-V spec.
> >
> > Thanks Gleb. Here is the link for the latest Hyper-V specification:
> > http://www.microsoft.com/en-us/download/details.aspx?id=39289
> >
> > This spec talks about how migration is handled with regards to TSC.
> >
> All I can see is "15.4.3 Partition Reference TSC Mechanism". I see
> nothing about HV_X64_MSR_TSC_FREQUENCY specifically. To access this MSR
> a partition needs AccessFrequencyMsrs privilege, may be partition that
> cab be migrated does not have this privilege?

If you use the MSR to read the TSC frequency, then indeed when you migrate then there
is no guarantee that the TSC frequency won't change when you migrate. The approach
described in section 15.4.3 is the preferred approach that can accommodate migration.

Regards,

K. Y 
> 
> --
> 			Gleb.
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