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Why does IOTLB invalidation take more than 1k cycles on average?

sammy17
Beginner
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I have been looking into IOMMU and IOTLB and noticed that the IOTLB invalidation operation in IOMMU takes many CPU clock cycles (average=1680, max=56090, min=516). 

Note that this is purely the time the OS kernel waits till a single IOTLB invalidation is completed by the IOMMU.

 

I went through the Intel VT-d specification to see a justifiable reason for IOTLB invalidation taking this many clock cycles. I saw that there are page structure caches that cache the intermediate memory accesses when doing a page table walk which should be invalidated when a relevant IOTLB entry gets invalidated. I can understand that this operation may take additional clock cycles.

However, I am not fully convinced that this is the only reason for IOTLB invalidation taking this many clock cycles on average.

Are there any other operations/scenarios that contribute towards the high cycle count  (or high latency) of IOTLB invalidation?

 

My setup:

CPU: Intel(R) Xeon(R) Silver 4114 CPU @ 2.20GHz

OS: Linux 6.2.8

 

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