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

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

On 2014/7/25 16:36, Jan Beulich wrote:
>>>> On 25.07.14 at 10:24, <yang.z.zhang@intel.com> 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.
>
> So then what's the patch good for? If it's just addressing a theoretical

No, this is not a theoretical problem.

I already reply to Yang, on BDW windows GFX driver really accesses this 
range now. Without this patch, the blue screen appears.

Tiejun

> issue, then this supports my intention to commit it only when the other
> (theoretical only) problem also gets taken care of.
>
> Jan
>
>
>