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Date: Sat, 26 Mar 2011 12:08:47 +0000
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Will Newton <will.newton@gmail.com>
Cc: Luke Kenneth Casson Leighton <luke.leighton@gmail.com>,
        linux-kernel@vger.kernel.org
Subject: Re: advice sought: practicality of SMP cache coherency implemented
 in assembler (and a hardware detect line)
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> Probably not. Is it a virtual or physical indexed cache? Do you have a
> precise workload in mind? If you have a very precise workload and you
> don't expect to get many write conflicts then it could be made to
> work.

I'm unconvinced. The user space isn't the hard bit - little user memory
is shared writable, the kernel data structures on the other hand,
especially in the RCU realm are going to be interesting.

> There are a number of mature cores out there that can do this already
> and can be bought off the shelf, I wouldn't underestimate the
> difficulty of getting your cache coherency protocol right particularly
> on a limited time/resource budget.

Architecturally you may want to look at running one kernel per device
(remembering that you can share the non writable kernel pages between
different instances a bit if you are careful) - and in theory certain
remote mappings.

Basically it would become a cluster with a very very fast "page transfer"
operation for moving data between nodes.