Re: [Xen-devel] [PATCH 1/2] x86/iommu: avoid mapping the interrupt address range for hwdom

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jul 25, 2019 at 10:22:17AM +0000, Jan Beulich wrote:
> On 23.07.2019 17:48, Roger Pau Monne wrote:
> > Current code only prevent mapping the lapic page into the guest
> > physical memory map. Expand the range to be 0xFEEx_xxxx as described
> > in the Intel VTd specification section 3.13 "Handling Requests to
> > Interrupt Address Range".
> > 
> > AMD also lists this address range in the AMD SR5690 Databook, section
> > 2.4.4 "MSI Interrupt Handling and MSI to HT Interrupt Conversion".
> > 
> > Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> I've committed this on the basis that it shouldn't hurt, but having
> thought about this some more I'm not really sure I see the point:
> The IOMMU special cases accesses into this range anyway, to redirect
> lookup to the interrupt remapping table instead of the DMA remapping
> one. Hence any mappings inserted into this range are simply useless,
> but shouldn't otherwise hurt.

Intel SDM contains:

"Software must ensure the second-level paging-structure entries are
programmed not to remap input addresses to the interrupt address
range. Hardware behavior is undefined for memory requests remapped to
the interrupt address range."

In section 3.13 (Handling Requests to Interrupt Address Range).

Since arch_iommu_hwdom_init/hwdom_iommu_map adds entries to both the
hap and the iommu page tables (or to hap only if shared) Xen should be
careful to not map this range because the iommu special cases this
range, but I'm not sure what hap does.

Thanks, Roger.

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