Re: [Xen-devel] [PATCH 2/4] iommu: generalize iommu_inclusive_mapping

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.08.18 at 11:20, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 01/08/2018 10:10, Jan Beulich wrote:
>>> Furthermore, on by default is the only reasonable option.  Dom0 is
>>> specifically given cpu mappings of the reserved regions, therefore it
>>> should have matching IOMMU mappings.
>> I don't think I can uniformly agree with this: There are reserved
>> regions which specifically would better not have IOMMU mappings:
>> LAPIC, IO-APIC, MMCFG just to name things I can immediately
>> think of.
> 
> Right, but that's not what I said.
> 
> For any CPU mapping, we should have a matching IOMMU mapping.  This is
> how hardware behaves in a native system, and is how all operating
> systems are written to behave.  Breaking this expectation will break
> software in unexpected ways.

Well, LAPIC is a clear exception to this rule, as from devices' point
of view the 0xFEExxxxx range has a completely different meaning.

> For the items you list there, we don't give dom0 CPU mappings (because
> restricting them is indeed a sensible thing), and the "matching" mapping
> in the IOMMU is also not present.

Not exactly: We still emulate those ranges for CPU side accesses,
while we don't emulate IOMMU / device side ones. So there's a
difference to native hardware anyway.

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel