Re: [v3][PATCH 1/1] xen:vtd: missing RMRR mapping while share EPT

["Chen, Tiejun" <tiejun.chen@intel.com>]

On 2014/7/25 16:28, Zhang, Yang Z wrote:
> Chen, Tiejun wrote on 2014-07-25:
>> On 2014/7/25 16:24, Zhang, Yang Z wrote:
>>> Jan Beulich wrote on 2014-07-25:
>>>>>>> On 24.07.14 at 19:12, <kevin.tian@intel.com> wrote:
>>>>>>> From: Chen, Tiejun
>>>>>> Sent: Thursday, July 24, 2014 4:00 AM
>>>>>>
>>>>>> intel_iommu_map_page() does nothing if VT-d shares EPT page table.
>>>>>> So rmrr_identity_mapping() never create RMRR mapping but in some
>>>>>> cases like some GFX drivers it still need to access RMRR.
>>>>>
>>>>> this is not accurate. as long as RMRR is reported, it's always
>>>>> necessary to have the identity mapping there, not just needed in
>>>>> SOME CASES. :-)
>>>>
>>>> Actually (as also said before) I think "in some cases" is quite correct:
>>>> There's no guarantee that a device will actually access the
>>>> region(s) an RMRR may specify for it. A particular example would be
>>>> the USB case where iiuc these regions are needed only until legacy
>>>> mode emulation
>> gets turned off.
>>>
>>> Yes, even for Intel GFX, the RMRR is never really accessed. So we never
>>> saw any issue even without this patch.
>>>
>>
>> Yang,
>>
>> Windows GFX driver really accesses this range now. Without this patch,
>> the blue screen appears.
>
> Okay, so for Linux guest, it works well even without having RMRR mapping.
>

Yes.

Tiejun