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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Friday, July 25, 2014 1:16 AM
> 
> >>> 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.
> 

the device may or may not access RMRR, but from virtualization p.o.v
as long as RMRR presents and you assign a device associated, we should
ALWAYS setup RMRR mapping since we don't know how device actually
work.

Thanks
Kevin

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