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

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

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.

> 
> Jan


Best regards,
Yang



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