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

[Jike Song]

On 02/23/2016 06:01 PM, Paolo Bonzini wrote:
> ----- Original Message -----
>> From: "Jike Song" <jike.song@xxxxxxxxx>
>> To: "Xiao Guangrong" <guangrong.xiao@xxxxxxxxxxxxxxx>
>> Cc: "Paolo Bonzini" <pbonzini@xxxxxxxxxx>, gleb@xxxxxxxxxx, mtosatti@xxxxxxxxxx, kvm@xxxxxxxxxxxxxxx,
>> linux-kernel@xxxxxxxxxxxxxxx, "kai huang" <kai.huang@xxxxxxxxxxxxxxx>, "Andrea Arcangeli" <aarcange@xxxxxxxxxx>,
>> "Kevin Tian" <kevin.tian@xxxxxxxxx>
>> Sent: Tuesday, February 23, 2016 4:02:25 AM
>> Subject: Re: [PATCH v3 00/11] KVM: x86: track guest page access
>>
>> +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.
> 

I may still misunderstand you, so apologize in advance ..

> I am not talking of DMA faults; I am talking of a guest that reads
> from disk into the PPGTT.

into PPGTT the page table itself? as said by Kevin in another mail,
this is NOT SUPPORTED.

> This is emulated DMA, and your approach of
> tracking guest page access from KVM means that you are not handling
> this.  Is this right?

Right, our tacking mechanism cares only CPU write, not Device write.

However, there is *NO* DMA emulation, just similar to passthrough.
The device(IGD) is only cable of r/w memory according the
shadowed PPGTT, which is managed by VGPU device-model, guaranteed
only memory that owned by this vgpu can be mapped. 

All we need is to track CPU writes from guest.

> If so, what happens if the guest does this
> kind of operation (for example because it is not using the PPGTT
> anymore)?  KVMGT should not be confused the next time it works on
> that PPGTT page.

As explained above, the device-model won't allow such things to happen.

> 
> Paolo
>

--
Thanks,
Jike