Re: [PATCH 3/3] x86/iommu: use __GFP_ZERO instead of memset for GART

[Joerg Roedel]

On Sat, Sep 27, 2008 at 08:18:55PM +0200, Ingo Molnar wrote:
> 
> another thing:
> 
> How hard would it be to add an CONFIG_IOMMU_DEBUG option that forces as 
> many DMA requests to go via the IOMMU as possible?
> 
> This slows things down of course so it's only for debugging - but it 
> also makes sure that we utilize the IOMMU code to the maximum - which is 
> not normally the case.
> 
> Would be nice to have it .config driven (default-disabled), so that 
> -tip's randconfig testing can stumble upon it every now and then. I've 
> got GART test-systems - this way we could find certain types of IOMMU 
> breakages sooner.

For AMD IOMMU I disabled the round-robin allocator to stress-test the
code. This means that the address allocation bitmap is always traversed
from the first bit. In consequence the TLB flushing is stressed a lot
(both in hardware and software) because the same DMA addresses are used
again and again. For testing I hardcoded it into the driver but I can
also make it depend on CONFIG_IOMMU_DEBUG.

Joerg

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