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: <20130918054335.GE5657@redhat.com>
Date:	Wed, 18 Sep 2013 08:43:35 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	KY Srinivasan <kys@...rosoft.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"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>,
	"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 Tue, Sep 17, 2013 at 07:53:36PM +0000, KY Srinivasan wrote:
> > > 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.
> 
Thanks.

--
			Gleb.
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