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Proposal for instruction set - now on Github
Author:  Date: 2016-09-06 16:19
I don't think you will be able to remove all usages of mmap(). Memory mapping is very convenient and if a microarchitecture doesn't allow for it won't be used much for general purpose computing. Programmers are already familiar with it and created a lot of utilities that use it. For example, you couldn't port most of the general purpose OSes to such an architecture, since they are based on virtual memory (there is a special fork of linux called uclinux [1] that doesn't need an MMU, though it's specialized for embedded solutions).

For example, 3D graphics APIs have a memory buffer map functions that are used to map part of GPU memory into the process address space. Since this part is already exclusive to the application (it has been allocated previously), there is no harm in bypassing the device driver (which performs the map) and it lets the user perform reads and writes with higher performance. A solution with a shadow buffer that is copied around when needed is possible, but would reduce performance.

A mechanism used by the Mill CPU family uses a protection data that maps which memory is to be accessible by which application [2]. It's stored in a PLB (protection look-aside buffer) that, unlike TLB, is not on a critical path since it only has to prevent a failing writes from affecting other CPUs and make failing reads cause a fault - no address translation is performed. Maybe such a mechanism would be a good compromise here? The OS (or any other entity) would be able to let another one access its memory on demand.

Mill also has a memory translation table, but the TLB sits outside of the core, near a memory controller. The mapping is global, not per-process (no TLB flush on context switch needed) and does the translation only when an access goes to the memory, at which point the latency is already high so making the TLB fast is not really that important. All the data in the caches is virtually addressed, virtually tagged.

But I repeat myself here. We could come up with a better solution instead, though it doesn't appear that it will be simple.

[1] https://en.wikipedia.org/wiki/%CE%9CClinux
[2] millcomputing.com/docs/memory/

 
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