Re: [PATCH] ARM: NOMMU: Fix exc_ret for stack frame type

[Jisheng Zhang]

On Mon, Oct 14, 2024 at 10:54:00AM +0100, Vladimir Murzin wrote:
> On 10/13/24 14:25, Jisheng Zhang wrote:
> > commit 72cd4064fcca ("ARM: 8830/1: NOMMU: Toggle only bits in
> > EXC_RETURN we are really care of") only sets BIT[3] for Thread mode
> > and BIT[2] for PSP, it leaves BIT[4] untouched. But there's such a
> > case: the pre-linux env makes use of FPU then the BIT[4] in 'lr' is
> > cleared, this brings an umatch issue since the NOMMU kernel doesn't
> 
> Can pre-linux env disable FPU before passing control to kernel (which
> is, as correctly pointed, doesn't know how to use FPU)?

IIRC, I did a experiment like this by clearing the SCB CPACR related
bits, but the stack frame type is still not correct. I searched in the
armv8m arm or cortex-mN's TRM I didn't find the relation between the
returned "lr" and the pre FPU usage.

What's more, IMHO, kernel needs to use the correct exc_ret no matter
the pre-linux env does, I.E if kernel/userspace supports FPU, then
clear BIT[4]; if no, set BIT[4].

PS: this is a regression: before the commit, the have-used-fpu pre-linux
env + linux nommu combination works; after the commit, it fails to
execute the init due to wrong stack frame type.

Thanks