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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.07.14 at 10:24, <yang.z.zhang@xxxxxxxxx> 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.

So then what's the patch good for? If it's just addressing a theoretical
issue, then this supports my intention to commit it only when the other
(theoretical only) problem also gets taken care of.

Jan


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