ARM64: Disabling warnings about deprecated armv8 instructions

[Eric Anholt]

Ard Biesheuvel <ard.biesheuvel at linaro.org> writes:

> On 30 January 2017 at 16:58, Russell King - ARM Linux
> <linux at armlinux.org.uk> wrote:
>> On Mon, Jan 30, 2017 at 02:50:23PM +0000, Ard Biesheuvel wrote:
> [..]
>>> I see. I was indeed mainly referring to userspace, which is what this
>>> thread is about, specifically setend and the CP15 barrier
>>> instructions. You can disagree with ARM's policy to deprecate and
>>> remove some instructions, but given the reality of it, I think it is
>>> perfectly reasonable to warn by default about setend instructions
>>> issued in 32-bit userspace when running under the 64-bit ARMv8 kernel.
>>
>> Linus has commented on this in the past, and his reaction (iirc) was
>> that deprecating instructions that userspace uses is not a sane
>> behaviour, pointing out that instructions that i386 supported still
>> work today on the latest Intel CPUs.
>>
>> He has a point, it's the same argument against breaking userspace
>> through making a change to the kernel.  It gives users a painful
>> experience.
>>
>> That said, no one expects that you can run an i386 binary on ARM,
>> which is pretty much understood by users.  However, ARM vs ARM64
>> where ARM64 provides a 32-bit ARM compatible environment is a
>> totally different story - it's _not_ compatible if it complains
>> about instructions that _were_ legal with 32-bit ARM binaries.
>> It makes me question the value of providing what is an incompatible
>> environment to run older binaries.
>>
>> The purpose of such an environment is to provide _compatibility_ to
>> older binaries.  You aren't providing a compatible environment if
>> you then decide to noisily warn about deprecated instructions.
>>
>> You might as well just rebuild the binaries for a 64-bit environment
>> and stop the illusion of a 32-bit compatible environment, because it
>> isn't.
>>
>
> Well, the question is really how to deal with 32-bit compatibility
> given the fact that the ARM architecture will drop such instructions
> in its next revision. Whether we agree with that, and whether we
> should argue our positions with the architects if we don't is not
> relevant here.
>
> What is relevant is whether we inform userspace if we spot such a
> deprecated instruction, and I think we should.
>
> Note that this is not only an issue for the arm64 kernel. The 32-bit
> kernel will have the exact same issue when you try to run it on
> hardware that no longer has support for those instructions.

It sounds like you should not expose a 32-bit compat environment on
future systems without all the 32-bit instructions.  I don't see why a
future processor not having compatibility means we need to break binary
compatiblity by default for today's processors.  And yes, spamming dmesg
is definitely breakage.
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