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Re: armhf kernels on arm64 hardware

On 2022-07-15 13:40 -0400, gene heskett wrote:
> On 7/15/22 12:16, Wookey wrote:
> > 
> > Clearly one normally does not run foreign-arch kernels on hardware so
> > we don't have to support it, and Ben is right to say 'this is not a
> > bug'.
> > 
> > On the other hand, if the armhf kernel does work on RPi4 with a few
> > config options, and there is an actual use case, then the question is
> > what is the downside of enabling the config options?
> It, LinuxCNC, does indeed run on an armhf kernel built right on the pi
> and has been since Jessie on a rpi3b.

And it is now in debian: https://tracker.debian.org/pkg/linuxcnc

> > Does this only work for the RPi4, or does it enable/prevent 32-bit kernels on other 64-bit machines?
> No. It runs with the same armhf kernel on an rpi3b, but the 3b is dragging
> its
> tongue on the floor where the 4b has some leftover zip.

Sorry. I meant other arm64 hardware than from broadcom (not other RPi flavours). I.e does enabling

Cause the kernel any issues on other platforms?

> Because our latency-test results are better on armhf than on arm64, we use
> armhf for its performance.

OK. How much better? What sort of performance difference are we talking about? 

And how many other users care about this? Debian is a general-purpose
OS and has to choose options that are generally useful or at least not
generally harmful. One user with some interesting hardware can clearly
install a new kernel built with specific options.

The question from Debian's POV is how many other people want to use
non-native arm kernels (and for what?). How many platforms is it
relevant to? And if there is a downside, how many does that effect,
and how/how much.

You say the kernel is 'a few kB bigger'. How many kB? Kernel size has
been critical on some armhf models in this past so even if that's the
only cost, it's not necessarily negligible. We may have dropped all
the platforms this was critical for by now, in which case perhaps a 'a
few kB' doesn't matter.

> > Do i386 kernels work on amd64 machines?
> Different architecture. No relevance here.

It's not entirely irrelevant. If it works on x86 then it's not
entirely unreasonable for people to expect it to work on arm. We do
strive for parity to the degree that it is possible and reasonable.
If it doesn't work on x86 then that justification can't be used, and
indeed strengthens the argument that 'just about nobody runs
non-native kernels - if you want to, you are on your own'.

Principal hats:  Debian, Wookware, ARM

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