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

[Alexander Graf]

> 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 :).

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
> 
>