[PATCH v4 04/12] ARM: entry: Fix iWMMXT TIF flag handling

[Ard Biesheuvel]

On Tue, 21 Mar 2023 at 20:19, Nicolas Pitre <npitre at baylibre.com> wrote:
>
> On Mon, Mar 20, 2023 at 2:19 PM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> > The conditional MOVS instruction that appears to have been added to test
> > for the TIF_USING_IWMMXT thread_info flag only sets the N and Z
> > condition flags and register R7, none of which are referenced in the
> > subsequent code.
>
> Really?
>
> As far as I know, the rsb instruction is a "reversed subtract" and that
> also sets the carry flag.
>
> And so does a move with a shifter argument (the last dropped bit is
> moved to the carry flag).
>
> What am I missing?
>

No, you are absolutely right. I looked up the wrong encoding in the
ARM ARM. MOVS without a shift preserves the carry flag, but the
variant you used here behaves as you describe.

So the code is correct - apologies for the noise.