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

On 2014/7/25 16:36, Jan Beulich wrote:
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

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.


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


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