From mboxrd@z Thu Jan  1 00:00:00 1970
From: Keir Fraser <keir@xen.org>
Subject: Re: [PATCH 3/5] vtdt: Modify vlapic code to add vtdt
	support
Date: Tue, 14 Dec 2010 08:48:51 +0000
Message-ID: <C92CDFF3.CCB9%keir@xen.org>
References: <F26D193E20BBDC42A43B611D1BDEDE7124EADAFCDE@shsmsx502.ccr.corp.intel.com>
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To: "Wei, Gang" <gang.wei@intel.com>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>
Cc: Tim Deegan <Tim.Deegan@eu.citrix.com>
List-Id: xen-devel@lists.xenproject.org

On 14/12/2010 08:22, "Wei, Gang" <gang.wei@intel.com> wrote:

>>> @@ -265,6 +265,7 @@ struct hvm_hw_lapic {
>>>      uint64_t             apic_base_msr;
>>>      uint32_t             disabled; /* VLAPIC_xx_DISABLED */
>>>      uint32_t             timer_divisor;
>>> +    uint64_t             tdt_msr;
>>>  };
>> 
>> Is this backward compatible with old HVM save images?
> 
> I am not sure about this. If it isn't, would you accept to simply add another
> data trunk for TDT msr?

That's Tim's call. I would personally prefer for Xen to accept truncated
chunks, and extend them with sensible 'old save image' defaults, such as
all-zeroes. That would be a generic solution to this case which will be
reusable in future, and avoid needlessly creating extra chunk types just for
backward compatibility reasons.

 -- Keir