Re: [PATCH v11 RESEND 9/9] arm64: armv8_deprecated: apply FEAT_LSUI for swpX emulation.

[Will Deacon]

On Mon, Dec 15, 2025 at 09:56:04AM +0000, Yeoreum Yun wrote:
> Hi,
> 
> > On Sun, 14 Dec 2025 11:22:48 +0000,
> > Yeoreum Yun <yeoreum.yun@xxxxxxx> wrote:
> > >
> > > Apply the FEAT_LSUI instruction to emulate the deprecated swpX
> > > instruction, so that toggling of the PSTATE.PAN bit can be removed when
> > > LSUI-related instructions are used.
> > >
> > > Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
> >
> > It really begs the question: what are the odds of ever seeing a CPU
> > that implements both LSUI and AArch32?
> >
> > This seems extremely unlikely to me.
> 
> Well, I'm not sure how many CPU will have
> both ID_AA64PFR0_EL1.EL0 bit as 0b0010 and FEAT_LSUI
> (except FVP currently) -- at least the CPU what I saw,
> most of them set ID_AA64PFR0_EL1.EL0 as 0b0010.

Just to make sure I understand you, you're saying that you have seen
a real CPU that implements both 32-bit EL0 *and* FEAT_LSUI?

> If you this seems useless, I don't have any strong comments
> whether drop patches related to deprecated swp instruction parts
> (patch 8-9 only) or not.
> (But, I hope to pass this decision to maintaining perspective...)

I think it depends on whether or not the hardware exists. Marc thinks
that it's extremely unlikely whereas you appear to have seen some (but
please confirm).

Will