Re: [PATCH 3/5] vtdt: Modify vlapic code to add vtdt support

[Keir Fraser <keir@xen.org>]

On 14/12/2010 09:13, "Wei, Gang" <gang.wei@intel.com> wrote:

> Keir Fraser wrote on 2010-12-14:
>>>>> @@ -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.
> 
> Ok. Let's wait for Tim to answer the call.

Well I propose the attached patch, and then you can use
hvm_load_entry_zeroextend() in vlapic.c. I would split this patch into two
pieces if I apply it (code movement first, then the zeroextend logic).

It needs an Ack from Tim however.

 -- Keir

> I just found some code in xen/hvm/save.h: _hvm_check_entry()
>     if ( type != d->typecode || len != d->length )
>     {
>         gdprintk(XENLOG_WARNING,
>                  "HVM restore mismatch: expected type %u length %u, "
>                  "saw type %u length %u\n", type, len, d->typecode,
> d->length);
>         return -1;
>     }
> 
> So I am assuming it would not be backward compatible with old HVM save images.
> 
> Jimmy