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

[Jann Horn]

[x86 folks in CC so that they can chime in on the precise rules for this stuff]

Hi!

I noticed that several makefiles under drivers/gpu/drm/amd/display/dc/
turn on floating-point instructions in the compiler flags
(-mhard-float, -msse and -msse2) in order to make the "float" and
"double" types usable from C code without requiring helper functions.

However, as far as I know, code running in normal kernel context isn't
allowed to use floating-point registers without special protection
using helpers like kernel_fpu_begin() and kernel_fpu_end() (which also
require that the protected code never blocks). If you violate that
rule, that can lead to various issues - among other things, I think
the kernel will clobber userspace FPU register state, and I think the
kernel code can blow up if a context switch happens at the wrong time,
since in-kernel task switches don't preserve FPU state.

Is there some hidden trick I'm missing that makes it okay to use FPU
registers here?

I would try testing this, but unfortunately none of the AMD devices I
have here have the appropriate graphics hardware...