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

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

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
> > > > 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
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