Re: [Qemu-devel] [PATCH v6 1/3] IOMMU: add option to enable VTD_CAP_CM to vIOMMU capility exposoed to guest

[Aviv B.D.]

On Fri, Nov 11, 2016 at 6:15 AM, Jason Wang <address@hidden> wrote:

>
>
> On 2016年11月11日 11:39, Michael S. Tsirkin wrote:
>
>> On Fri, Nov 11, 2016 at 10:32:42AM +0800, Jason Wang wrote:
>>
>>>
>>> On 2016年11月10日 06:00, Michael S. Tsirkin wrote:
>>>
>>>> On Wed, Nov 09, 2016 at 03:28:02PM +0800, Jason Wang wrote:
>>>>
>>>>>
>>>>>> On 2016年11月08日 19:04, Aviv B.D wrote:
>>>>>>
>>>>>>> From: "Aviv Ben-David"<address@hidden>
>>>>>>>>
>>>>>>>> This capability asks the guest to invalidate cache before each map
>>>>>>>> operation.
>>>>>>>> We can use this invalidation to trap map operations in the
>>>>>>>> hypervisor.
>>>>>>>>
>>>>>>> Hi:
>>>>>>
>>>>>> Like I've asked twice in the past, I want to know why don't you cache
>>>>>> translation faults as what spec required (especially this is a guest
>>>>>> visible
>>>>>> behavior)?
>>>>>>
>>>>>> Btw, please cc me on posting future versions.
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>> Caching isn't guest visible.
>>>>
>>> Seems not, if one fault mapping were cached by IOTLB. Guest can notice
>>> this
>>> behavior.
>>>
>> Sorry, I don't get what you are saying.
>>
>> Spec just says you*can*  cache,
>>>> not that you must.
>>>>
>>>> Yes, but what did in this patch is "don't". What I suggest is just a
>>> "can",
>>> since anyway the IOTLB entries were limited and could be replaced by
>>> other.
>>>
>>> Thanks
>>>
>> Have trouble understanding this. Can you given an example of
>> a guest visible difference?
>>
>
> I guess this may do the detection:
>
> 1) map iova A to be non-present.
> 2) invalidate iova A
> 3) map iova A to addr B
> 4) access iova A
>
> A correct implemented CM may meet fault in step 4, but with this patch, we
> never.
>
>
By the specification (from intel) section 6.1 (Caching mode) tells that
this bit may be present
only on virtual machines. So with just this point the guest can detect that
it running in
a VM of some sort. Additionally, the wording of the specification enable
the host to issue a fault
it your scenario but, doesn't requiring it.

Thanks,
Aviv.