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Re: Summary of the Arm ports BoF at DC17




On 2017-09-15, Ben Hutchings wrote:
> On Thu, 2017-09-14 at 03:40 +0100, Steve McIntyre wrote:
> [...]
>> There is optional kernel support to trap the exceptions here
>> and emulate the instructions, but it's really not recommended for
>> serious use (e.g. on a build machine!).
> [...]
>
> Why is it not recommended?  Terrible performance, or known bugs?

On arm64 kernels building armhf packages for the reproducible builds
builders, I'm seeing hundreds or even thousands of kernel log warnings
per second when building anything that makes use of
CP15_BARRIER_EMULATION (e.g. anything using ghc):

  [85740.553537] cp15barrier_handler: 115316 callbacks suppressed
  [85740.559358] "ghc-pkg" (29572) uses deprecated CP15 Barrier instruction at 0xf5beaa7c
  [85740.567344] "ghc-pkg" (29572) uses deprecated CP15 Barrier instruction at 0xf5beaa9c


That *might* be sufficient to actually impact performance; it certainly
makes it hard to read other kernel log messages on those machines...


There was a bug reported on ghc related to this:

  https://bugs.debian.org/864847


live well,
  vagrant

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