ARM caches variants.

[Gilles Chanteperdrix]

Catalin Marinas wrote:
> On Tue, 2010-03-23 at 12:39 +0000, Gilles Chanteperdrix wrote:
>> As I understood, the VIVT cache has the following issues:
>> - issue #1; two processes may use the same virtual address for different
>> physical addresses, but will share the same cache line and see memory
>> corruption if no precaution is taken;
>> - issue #2; two processes may use different virtual addresses for the
>> same physical address (shared mapping), but will use different cache
>> lines, causing all sorts of incoherence if no precaution is taken;
>> - issue #3; the same process may use different virtual addresses for the
>> same physical address (same shared mapping mapped several time in the
>> same process virtual memory), basically almost the same issue as issue #2.
> [...]
>> Now, if we look at VIPT, aliasing caches:
>> - the physical tagging solves issue #1 automatically,
>> - the cache colouring technique used in arch_get_unmapped_area solves
>> issue #2 and #3 by ensuring that the areas using the same physical
>> address will end up using the same cache lines, and avoid aliases "by
>> construction".
> [...]
>> First question: did I get it right?
> 
> Yes.
> 
>> Second question: do issue #1, #2 and #3 have official non-ambiguous names?
> 
> I don't think there are any official names. You could say cache aliasing
> though not sure it covers everything.
> 
>> Now, the stupid question: why not using the cache colouring technique
>> used for VIPT caches to solve issue #3 with VIVT caches?
> 
> Because with aliasing VIPT it is guaranteed that if a virtual address
> has the same offset in a 16KB block (i.e. the same colour - there are
> only 4 colours given by bits 13 and 12 of the virtual address), you get
> the same cache line allocated for a given physical address. The tag of a
> cache line is given by bits 31..14 of the physical address.
> 
> With VIVT, the cache tags are not aware of the physical address, hence
> you can have 2^20 colours (bits 31..12 of the virtual address). You
> would need to map a physical address at the same virtual address in all
> applications sharing it (and you may end up with uClinux :)).

Ok. I do not get it. Let us do it in slow motion: as I understand, the
problem with issue #2 and #3 is not really about the tag, but about two
different virtual addresses ending up using different cache lines,
whatever the tag. By using cache colouring, can not we ensure that they
end up in the same cache line and simply evict each other because they
do not have the same tag?

In other word, is not the cache line used by virtual address addr:
(addr % cache size) / (cache line size)
?

Or is it precisely what armv6 guarantees?

-- 
					    Gilles.