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ARM's multiply-mapped memory mess

ARM's multiply-mapped memory mess

Posted Oct 14, 2010 6:42 UTC (Thu) by dlang (subscriber, #313)
Parent article: ARM's multiply-mapped memory mess

warning for one cycle will do very little good. it generally takes longer than that for any distros to start using a kernel and get it into the hands of the users (and in the embedded space where ARM is so strong, it's even worse)

but even if it only took a week for a distro to QA and ship a kernel, and users started putting pressure on immediately, there is no chance for any development work to take place and still make it into the mainline kernel before the merge window closes.

it will probably take multiple kernel cycles before the warning is seen by any users who don't compile their own kernels, and then more time for the pressure from the users to build up, and then development time to fix the problem, and then time to merge the fixes upstream.

I don't see this taking place in less than a year, and probably a lot longer before everything is fixed.

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ARM's multiply-mapped memory mess

Posted Oct 14, 2010 13:24 UTC (Thu) by NAR (subscriber, #1313) [Link]

In a similar vein: my impression from the article is that there's been a memory corruption bug on the ARM architecture for at least half a year, yet nobody has been actually bit by it. Is there anybody who's actually using this kernel on version 6 ARM hardware?

ARM's multiply-mapped memory mess

Posted Oct 14, 2010 15:00 UTC (Thu) by marcH (subscriber, #57642) [Link]

The other possibility is that the behaviour "undefined" in the spec is actually very well defined in practice.

ARM's multiply-mapped memory mess

Posted Oct 14, 2010 15:19 UTC (Thu) by felixfix (subscriber, #242) [Link]

This puzzles me too. I guessed as marcH does, in the first answer to your question. It sure sounds spooky, but if it's been running this way for some time (years?), it must be one of those things that the manufacturer might want to use themselves someday for something different, so they put out a vague "undefined" warning while knowing that it does work.

ARM's multiply-mapped memory mess

Posted Oct 15, 2010 12:53 UTC (Fri) by drag (subscriber, #31333) [Link]

One of the problems your going to face with ARM is that there is no one single implementation of the hardware, nor all the available hardware now is going to reflect all the hardware that is going to be.

ARM 6 is essentially a specification for a processor and not a real processor. Using the memory in the way the kernel does is 'unspecified'. It could work on today's processors made by Ti, but it could completely backfire on tomorrow's processors made by Marvel.

It's impossible to know and if it does start corrupting memory in the kernel then it's going to be 100% ok as far as the processor designers are concerned because they are still following the specification.

It's similar to having the kernel rely on unspecified GCC features were once a user chooses a GCC version they are forced to use it for ever and cannot change it no matter how badly it works with Linux.

ARM's multiply-mapped memory mess

Posted Oct 15, 2010 18:34 UTC (Fri) by dlang (subscriber, #313) [Link]

that depsnds on exactly how detailed the specs are. I believe that the ARM specs are not behavior (i.e. must implement these commands these ways), but are instead a much lower level (arrange logic gates in this way)

ARM's multiply-mapped memory mess

Posted Oct 15, 2010 22:17 UTC (Fri) by gnb (subscriber, #5132) [Link]

That in turn depends on the licence: most of their customers (sorry, partners) licence an implementation. They get given either synthesizable code or a hard macro that implements the core (+MMU +cache as appropriate) to drop into their chip. Modulo bugs, all chips of this sort with the same core IP should behave the same. A few large partners (Marvell, Ti?) have architecture licences that cover changing the implementation provided it still matches the spec. . In those cases all bets are off for behaviour that the spec. doesn't define.

ARM's multiply-mapped memory mess

Posted Oct 17, 2010 15:58 UTC (Sun) by oak (guest, #2786) [Link]

If it triggers user visible issues only rarely, in seemingly random circumstances (like some of the memory corruption bugs do) i.e. it's not reproducible, it's very well possible that nobody's identified what's causing "all those crashes", or that they're related.

ARM's multiply-mapped memory mess

Posted Oct 18, 2010 20:19 UTC (Mon) by mwh (subscriber, #582) [Link]

It seems to be the cause of this bug, fwiw:

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