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

[Vladimir Murzin <vladimir.murzin@arm.com>]

On 10/15/24 01:02, Jisheng Zhang wrote:
> 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
> 

Fair enough.

Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>

Please, upload the patch in RMK's patch system [1]

[1] https://www.arm.linux.org.uk/developer/patches/


Vladimir