ARM caches variants.

[jamie@shareable.org (Jamie Lokier)]

Catalin Marinas wrote:
> > In other word, is not the cache line used by virtual address addr:
> > (addr % cache size) / (cache line size)
> 
> With any cache line, you have an index and a tag for identifying it. The
> cache may have multiple ways (e.g. 4-way associative) to speed up the
> look-up. For a 32KB 4-way associative cache you have 8KB per way (2^13).
> 
> If the cache line size is 32B (2^5), the index of a cache line is:
> 
> addr & (2^13 - 1) >> 5
> 
> e.g. bits 12..5 from the VA are used for indexing the cache line.
> 
> The tag is given by the rest of the top bits, in the above case bits
> 31..13 of the VA (if VIVT cache) or PA (VIPT cache).
> 
> The cache look-up for a VA goes something like this:
> 
>      1. extracts the index. With a 4-way associative cache there are 4
>         possible cache lines for this index
>      2. extracts the tag (from either VA or PA, depending on the cache
>         type). For VIPT caches, it needs to do a TLB look-up as well to
>         find the physical address
>      3. check the four cache lines identified by the index at step 1
>         against their tag
>      4. if the tag matches, you get a hit, otherwise a miss
> 
> For your #2 and #3 issues, if two processes map the same PA using
> different VAs, data can end up pretty much anywhere in a VIVT cache. If
> you calculate the index and tag (used to identify a cache line) for two
> different VAs, the only common part are bits 11..5 of the index (since
> they are inside a page). If you want to have the same index and tag for
> the two different VAs, you end up with having to use the same VA in both
> processes.
> 
> With VIPT caches, the tag is the same for issues #2 and #3. The only
> difference may be in a few top bits of the index. In the above case,
> it's bit 12 of the VA which may differ. This gives you two page colours
> (with 64KB 4-way associative cache you have 2 bits for the colour
> resulting in 4 colours).

That's a very helpful explanation, thank you.

Am I to understand that "VIPT aliasing" means there are some of those
bits and therefore >= 2 colours, and "VIPT non-aliasing" means the
cache size / ways is <= PAGE_SIZE, and therefore has effectively 1 colour?

Or does "non-aliasing" mean something else?

I suspect some x86s have VIPT caches, especially AMD (I've seen timing
measurements which clearly show page colour effects), and I can only
imagine that aliasing is prevent by when a cache line requests to be
filled from higher level cache (L2), something very similar to SMP
MESI cache coherence gets involved to keep both lines consistent.

That would make a "VIPT non-aliasing" cache that has multiple colours.
Is that ever done on the ARM architecture?

Thanks again,
-- Jamie