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Re: [Xen-devel] [PATCH 3/4 V3] X86: MPX IA32_BNDCFGS msr handle

Tim Deegan wrote:
> At 11:12 +0000 on 28 Nov (1385633520), Liu, Jinsong wrote:
>> Tim Deegan wrote:
>>> At 03:17 +0000 on 28 Nov (1385605079), Liu, Jinsong wrote:
>>>> Andrew Cooper wrote:
>>>>> On 27/11/13 15:02, Liu, Jinsong wrote:
>>>>>> Andrew Cooper wrote:
>>>>>>> It is very common to have pools of servers made of different
>>>>>>> generations of CPU.  E.g. Ivy Bridge and Haswell.  To safely
>>>>>>> migrate a VM, the feature set the VM can see must be the common
>>>>>>> subset of the two. 
>>>>>>> ~Andrew
>>>>>> Yes -- but that's not a reason to prevent MPX feature (or, any
>>>>>> new features) -- otherwise you have to prevent any new features.
>>>>>> The right place to control cpuid policy of a pool is at higher
>>>>>> level, where it has full information of the pool machines and so
>>>>>> it's right place to make decision what cpuid feature set would
>>>>>> be proper for the specific pool. 
>>>>> That is exactly a reason to prevent MPX.
>>>>> If the domain cpuid policy (which is set by the toolstack) states
>>>>> that MPX should be disabled, then MPX must be hidden from the HVM
>>>>> guest, even if the hardware supports MPX.
>>>> No. That's _not_ a reason to prevent MPX -- toolstack still has the
>>>> right to disable MPX, no matter h/w support MPX or not. Refer
>>>> xc_cpuid_set().
>>> There seems to be a lot of confusion here.  As far as I can tell,
>>> the only sensible mechanism is: 
>>> - If the hardware doesn't support MPX, mask it in guest CPUID.
>>> - If the domain cpuid policy masks the MPX feature, disable it.
>>> - Otherwise, enable it, and advertise it in guest CPUID.
>>> In any case, the CPUID fields seen by the guest _must_ match
>>> whether the feature is available. 
>>> Tim.
>> Do you mean xc_cpuid_set() is some confusion? Yes, it's some buggy
>> that need got fix at tools side. 
>> I take it here as an example just indicate 'toolstack has the right
>> to disable/mask hardware feature, if it want to do so per domain
>> cpuid policy'.  
> In that case, I think you and Andrew are agreeing with each other. :)
> The important detail is that if the toolstack has disabled the feature
> using the CPUID policy, the hypervisor should not expose the feature
> to the guest.
> Cheers,
> Tim.

Yeah, exactly :)

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