AMD DC graphics display code enables -mhard-float, -msse, -msse2 without any visible FPU state protection

[Peter Zijlstra]

On Fri, Apr 17, 2020 at 04:27:28PM -0400, Rodrigo Siqueira wrote:
> I'm going to work on the FPU issues in the display code. In this sense,
> could you clarify a little bit more about the Makefile issues?

I think it might have been PEBKAC, I assumed allmodconfig would end up
selecting the driver, it doesn't!

Adding "AMD GPU" to a defconfig seems to make it work:

$ make O=defconfig-build/ drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o

Sorry about the confusion.

> Also, I applied the patch `[PATCH v4] x86: insn: Add insn_is_fpu()` and
> tried to reproduce the warning that you described but I didn't see it.
> Could you explain to me how I can check those warnings?

grab:

  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/fpu

Then build like above:

$ make O=defconfig-build/ drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o

And manually validate:

$ defconfig-build/tools/objtool/objtool check -Ffa defconfig-build/drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o
defconfig-build/drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o: warning: objtool: dcn_validate_bandwidth()+0x1b17: FPU instruction outside of kernel_fpu_{begin,end}()