Re: Implications of QEMU binfmt transparent emulation for builds

[Mark H Weaver <mhw@netris.org>]

Hi Christopher,

Christopher Baines <mail@cbaines.net> writes:

> I'm starting to play with mixing native and emulated builds with the
> Guix Build Coordinator again. I did do this many months ago, but at that
> time, there wasn't support for targeting retries across a range of
> machines, to help avoid blockages due to QEMU issues.
>
> Anyway, something that's been on my mind regarding QEMU and builds is
> how well this matches up with building natively.

Here's one significant problem with emulated builds, although perhaps
not the kind of problem you were asking about:

Due to differences in the memory models of Intel and other systems such
as ARM, it is easy to write multithreaded code that works robustly on
Intel systems but sporadically fails on other architectures such as ARM.

However, an ARM emulator running on an Intel system will effectively use
an Intel memory model, because it would be prohibitively expensive to
faithfully simulate the ARM memory model on Intel hardware, and the only
benefit would be to demonstrate bugs that can't happen on Intel systems.

Therefore, test suites run under an emulator will almost certainly fail
to find architecture-specific thread safety problems, which I expect are
among the most important tests to be done, and among the most likely to
otherwise go unnoticed.

For this reason, I suggest that we should avoid emulated builds in our
build farm.

     Regards,
       Mark