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

Andrew Cooper wrote:
> On 27/11/13 15:02, Liu, Jinsong wrote:
>> Andrew Cooper wrote:
>>> On 27/11/13 14:37, Liu, Jinsong wrote:
>>>> Andrew Cooper wrote:
>>>>> On 27/11/13 14:27, Liu, Jinsong wrote:
>>>>>> Andrew Cooper wrote:
>>>>>>> On 27/11/13 13:50, Liu, Jinsong wrote:
>>>>>>>> From 291adaf4ad6174c5641a7239c1801373e92e9975 Mon Sep 17
>>>>>>>> 00:00:00 2001 From: Liu Jinsong <jinsong.liu@xxxxxxxxx>
>>>>>>>> Date: Thu, 28 Nov 2013 05:26:06 +0800
>>>>>>>> Subject: [PATCH 3/4 V3] X86: MPX IA32_BNDCFGS msr handle
>>>>>>>> When MPX supported, a new guest-state field for IA32_BNDCFGS
>>>>>>>> is added to the VMCS. In addition, two new controls are added:
>>>>>>>>  - a VM-exit control called "clear BNDCFGS"
>>>>>>>>  - a VM-entry control called "load BNDCFGS."
>>>>>>>> VM exits always save IA32_BNDCFGS into BNDCFGS field of VMCS.
>>>>>>>> Signed-off-by: Xudong Hao <xudong.hao@xxxxxxxxx>
>>>>>>>> Reviewed-by: Liu Jinsong <jinsong.liu@xxxxxxxxx>
>>>>>>>> Unlikely, but in case VMX support is not available, not expose
>>>>>>>> MPX to hvm guest.
>>>>>>> You are still missing the point.
>>>>>>> I as the administrator choose to prevent an HVM guest from using
>>>>>>> MPX. Perhaps I want to create a heterogeneous pool.
>>>>>>> Therefore, the bit is disabled in the domains cpuid policy,
>>>>>>> despite being available on the hardware.
>>>>>>> ~Andrew
>>>>>> Could you tell me the reason why choose to prevent HVM from
>>>>>> using MPX? 
>>>>>> Thanks,
>>>>>> Jinsong
>>>>> For exactly the case I gave - a VM in a heterogeneous pool where
>>>>> one server supports MPX and the other is lacking the MPX feature.
>>>>> ~Andrew
>>>> I didn't see the point of your case to prevent HVM MPX feature.
>>>> Could you elaborate more of your concern?
>>>> Thanks,
>>>> Jinsong
>>> 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.   
>> Thanks,
>> Jinsong
> 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.
> ~Andrew

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().

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