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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Roger 
> Pau Monné
> Sent: 25 July 2019 11:54
> To: Jan Beulich <JBeulich@xxxxxxxx>
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH 1/2] x86/iommu: avoid mapping the interrupt 
> address range for hwdom
> 
> 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.

Presumably such ranges should never end up in the P2M as they'd need to be 
trapped for emulation?

  Paul

> 
> Thanks, Roger.
> 
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