Re: [PATCH v3 00/11] KVM: x86: track guest page access

[Paolo Bonzini]

On 23/02/2016 06:44, Tian, Kevin wrote:
>> From: Song, Jike
>> Sent: Tuesday, February 23, 2016 11:02 AM
>>
>> +Kevin
>>
>> On 02/22/2016 06:05 PM, Xiao Guangrong wrote:
>>>
>>> On 02/19/2016 08:00 PM, Paolo Bonzini wrote:
>>>>
>>>> I still have a doubt: how are you going to handle invalidation of GPU
>>>> shadow page tables if a device (emulated in QEMU or even vhost) does DMA
>>>> to the PPGTT?
>>>
>>> I think Jike is the better one to answer this question, Jike, could you
>>> please clarify it? :)
>>>
>>
>> Sure :)
>>
>> Actually in guest PPGTT is manipulated by CPU rather than GPU. The
>> PPGTT page table itself are plain memory, composed & modified by the
>> GPU driver, i.e. by CPU in Non-Root mode.
>>
>> Given that, we write-protected guest PPGTT, when VM writes PPGTT, EPT
>> violation rather than DMA fault happens.
> 
> 'DMA to PPGTT' is NOT SUPPORTED on our vGPU device model. Today
> Intel gfx driver doesn't use this method, and we explicitly list it as a
> guest driver requirement to support a vGPU. If a malicious driver does 
> program DMA to modify PPGTT, it can only modify guest PPGTT instead
> of shadow PPGTT (being guest invisible). So there is no security issue 
> either.

Ok, thanks for confirming.

Paolo