[fulafel]

Are the memory semantics programmer-friendly on all (or most) platforms when using aliasing mappings like this? Eg if two CPUs concurrnetly update the memory through different VM address aliases, is the observed semantics identical vs accessing through only one address or not?

[gpderetta]

I think POSIX requires it to work. On all sane architectures that use physically indexed caches (or VIPT) that's easy. On other architectures the OS has to bend over backward to preserve the fiction.

[crest]

VIPT caches only allow this if all virtual aliases for the physical memory backing them map to the same cache set. Let's look at an example: Writing to the first cache line in both aliases of such a double mapped ring buffer. The data cache indexes the cache by the virtual addresses while the MMU translates them. If the virtual addresses have the correct cache coloring to always hit the same set of cache to compare the physical address to the tag it works, but if virtual addresses don't index to the same set of cache lines you get data corruption because there will be two dirty cache lines with conflicting data for the same physical address waiting to be written back in an unknowable order. Good luck debugging this should your system allow you to create such a memory mapping. At least Linux and FreeBSD used to allow you to setup this time bomb on ARMv5 and you have only two bad options to choose from as kernel: break software relying on this or make it unbearable slow by making aliased memory uncacheable.

[FullyFunctional]

Nobody (not even IBM AFAIK) uses coloring anymore so this is moot. ARMv5 isn't even supported by Linux anymore.

All modern caches PIPT, VIPT, or even VIVT must work with this scheme as it's semantically transparent to virtual memory. The performance of line-crossing accesses is a totally different issue and I would never used this "trick".

[gpderetta]

You mean virtually indexed caches. VIPT is a transparent improvement on physically indexed caches, which is used by most architectures today.

Otherwise, yes, that's what I mean by the OS bending over backward.

Linus has choice words for such architectures.