AMD IOMMU [PSARC/2008/561]

[Garrett D'Amore]

Vikram Hegde wrote:
> Hi,
>
>> Abilities to disable the IOMMU as a choice? Performance impact on 
>> various systems and interactions with other projects that have gone 
>> to some length to work within S/G and its performance 
>> characteristics. How will this affect them? Cache effects? 
>> Interactions with some VM work currently being undertaken?
>>
>> I can't imagine such a major change being closed/approved/automatic. 
> Performance is driver dependant. There is no way a single project team 
> can test the gamut of drivers out there and certify that the IOMMU 
> will  give acceptable performance for all needs (which is itself a 
> subjective opinion). We have always planned to provide the ability to 
> disable the IOMMU on a per driver basis. Does that address your 
> performance concerns ?

Actually, for many cases, IOMMU may *improve* performance.  This is 
because it will make it easier to satisfy device constraints for DMA 
resources without requiring the use of a bounce buffer or dma windows.

That said, drivers which assume that setup and tear down of DMA is 
"cheap" may take a perf. hit when using an IOMMU.  This is already the 
case for SPARC systems, btw.

    -- Garrett
>
> Vikram
>
>
> Matthew Jacob wrote:
>> On 09/03/08 16:34, Jerry Gilliam wrote:
>>> I am sponsoring the following fast-track for Vikram Hegde.  This
>>> case presents an overview and architectural details on an IOMMU
>>> for AMD cpus.  This information is being presented for information
>>> only, for discussion and future reference.  As there's nothing to
>>> approve, I believe Closed Approved Automatic is appropriate.
>>>
>>>
>>>
>>>   
>>
>> Abilities to disable the IOMMU as a choice? Performance impact on 
>> various systems and interactions with other projects that have gone 
>> to some length to work within S/G and its performance 
>> characteristics. How will this affect them? Cache effects? 
>> Interactions with some VM work currently being undertaken?
>>
>> I can't imagine such a major change being closed/approved/automatic.
>