linux-kernel - Re: [PATCH v11 RESEND 9/9] arm64: armv8_deprecated: apply FEAT

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Message-ID: <aW5O714hfl7DCl04@willie-the-truck>
Date: Mon, 19 Jan 2026 15:34:07 +0000
From: Will Deacon <will@...nel.org>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: Marc Zyngier <maz@...nel.org>, catalin.marinas@....com,
	broonie@...nel.org, oliver.upton@...ux.dev, miko.lenczewski@....com,
	kevin.brodsky@....com, ardb@...nel.org, suzuki.poulose@....com,
	lpieralisi@...nel.org, yangyicong@...ilicon.com,
	scott@...amperecomputing.com, joey.gouly@....com,
	yuzenghui@...wei.com, pbonzini@...hat.com, shuah@...nel.org,
	mark.rutland@....com, arnd@...db.de,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v11 RESEND 9/9] arm64: armv8_deprecated: apply FEAT_LSUI
 for swpX emulation.

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@....com> 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@....com>
> >
> > 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