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

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

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@xxxxxxxxx> 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.

> 
> Tiejun


Best regards,
Yang



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