RE: [Xen-devel] [PATCH 3/5] vtdt: Modify vlapic code to add vtdt support

["Wei, Gang" <gang.wei@xxxxxxxxx>]

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.

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

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