Why does Intel use a VIPT cache and not VIVT or PIPT?

Question

I am not sure, but if I remember right Intel uses a VIPT (virtual index physical tagged) cache, I would like to know the reason of this choice, why is it better than VIVT or PIPT, what advantages does it procure and maybe what disadvantages. Thank you.

Answer 1

The exact design decisions are probably not published, but in general the benefits for VIPT are :

1. Virtual indexing means you can start reading the set from the cache before (or in parallel with) looking up the translation in the TLB. This means that the common case lookup would be done much faster than a physically indexed cache, where you can only lookup the cache after you have the translation.
2. Physical tagging allows you to avoid aliasing - both in cases where a single virtual address maps to several physical ones (e.g. different threads overlapping in virtual addr ragne), or multiple virtual addresses map to the same physical line (e.g. inter-thread communication). Both cases may cause loss of coherency in a virtually tagged cache and would require complicated mechanisms for detection.

Answer 2

"vivt pipt" is a commonly used mapping scheme in computer caches. It stands for "Virtual Index, Virtual Tag, Physically Indexed Physically Tagged". In this mapping scheme, cache blocks are indexed by virtual addresses and tagged with virtual addresses, but the indexing is performed physically and the tags are stored physically.