ARM64: Disabling warnings about deprecated armv8 instructions

[Michael Zoran]

On Mon, 2017-01-30 at 18:17 +0000, Måns Rullgård wrote:
> Michael Zoran <mzoran at crowfest.net> writes:
> 
> > On Mon, 2017-01-30 at 17:09 +0000, Ard Biesheuvel wrote:
> > > 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.
> > 
> > Just to inject two cents worth since I'm still on the thread and I
> > originally started the topic...
> > 
> > The Intel comparison seems relevant.  I think in theory you can
> > still
> > run old MS-DOS software on an Intel PC if you were so inclined to
> > do
> > so.
> > 
> > I would think the best would be to turn the warnings off by default
> > when running in 32 bit mode with the option to turn them back on. 
> > Then
> > as time progresses more of more of these instructions can be
> > converted
> > to software emulation and removed from the hardware.  And hopefully
> > as
> > time progresses, all the user mode software gets upgraded to 64 bit
> > so
> > that 32 bit get dropped.
> > 
> > The issue seems to me to be more that the kernel is screaming in
> > the
> > logs rather then if it's emulating the instructions in hardware or
> > software.
> 
> Might warning once be an acceptable compromise?

Absolutely!