Re: [Qemu-devel] [PATCH 0/2] intel_iommu: Add support for translation for devices behind bridges

[Knut Omang]

On Tue, 2014-10-21 at 11:07 +0200, Alexander Graf wrote:
> 
> 
> > Am 21.10.2014 um 07:26 schrieb Knut Omang <address@hidden>:
> > 
> >> On Tue, 2014-10-21 at 01:29 +0200, Alexander Graf wrote:
> >> 
> >> 
> >>> Am 21.10.2014 um 00:34 schrieb Knut Omang <address@hidden>:
> >>> 
> >>> This patch set changes the data structure used to handle address spaces 
> >>> within
> >>> the emulated Intel iommu to support traversal also if bus numbers are 
> >>> dynamically
> >>> allocated, as is the case for devices that sit behind root ports or 
> >>> downstream switches.
> >>> This means that we cannot use bus number as index, instead a QLIST is 
> >>> used.
> >>> 
> >>> This requires a change in the API for setup of IOMMUs which is taken care 
> >>> of by 
> >>> the first patch. The second patch implements the fix.
> >> 
> >> Are you sure that this works on real hardware? How does that one
> >> communicate sub-bridge liodns to the iommu? How do they get indexed
> >> from software?
> > 
> > I do not claim to fully understand the details of how this is
> > implemented in hardware, but I believe the implementation I propose here
> > should be functionally equivalent to what the Intel IOMMU offers, and
> > similar to the original implementation here, except that the data
> > structure is valid also before enumeration when behind buses.
> 
> Can you please give me a pointer to the vt-d spec's section that explains 
> iommu behavior behind bridges?
> 
> I've also added Alex W who has played with PCI bridges behind iommus quite a 
> bit recently.
> 
> > 
> > After enumeration, the only difference would be that during
> > invalidation, there is a list search for the right bus rather than an
> > index lookup as before, slightly less efficient but at the benefit of
> > being independent of bus numbering during setup.
> 
> I don't think the implementation is bad, I'm just not sure that it follows 
> the spec, 
> so I want to confirm :).

http://www.intel.com/content/dam/www/public/us/en/documents/product-specifications/vt-directed-io-spec.pdf

Knut

> Alex
> 
> > 
> > Wrt the currently implemented IOMMUs for other architectures, they were
> > all ignoring the bus argument anyway, so the API change did not make
> > much difference.
> > 
> > Knut
> > 
> >> Alex
> > 
> >