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

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Message-ID: <aW/Ck3M3Xg02DpQX@e129823.arm.com>
Date: Tue, 20 Jan 2026 17:59:47 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: Will Deacon <will@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>, 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, 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 Tue, Jan 20, 2026 at 10:07:33AM +0000, Yeoreum Yun wrote:
> > Hi Mark,
> >
> > > On Mon, Jan 19, 2026 at 10:32:11PM +0000, Yeoreum Yun wrote:
> > > > > On Mon, Dec 15, 2025 at 09:56:04AM +0000, Yeoreum Yun wrote:
> > > > > > > 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).
> > > >
> > > > What I meant was not a 32-bit CPU with LSUI, but a CPU that supports
> > > > 32-bit EL0 compatibility (i.e. ID_AA64PFR0_EL1.EL0 = 0b0010).
> > > > My point was that if CPUs implementing LSUI do appear, most of them will likely
> > > > continue to support the existing 32-bit EL0 compatibility that
> > > > the majority of current CPUs already have.
> > >
> > > That doesn't really answer Will's question. Will asked:
> > >
> > >   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?
> > >
> > > IIUC you have NOT seen any specific real CPU that supports this, and you
> > > have been testing on an FVP AEM model (which can be configured to
> > > support this combination of features). Can you please confirm?
> > >
> > > I don't beleive it's likely that we'll see hardware that supports
> > > both FEAT_LSUI and AArch32 (at EL0).
> >
> > Yes. I've tested in FVP model. and the latest of my reply said
> > I confirmed that Marc's and your view was right.
>
> It's probably still worth adding something to the cpufeature stuff to
> WARN() if we spot both LSUI and support for AArch32.
>

On second thought, while a CPU that implements LSUI is unlikely to
support AArch32 compatibility,
I don't think LSUI requires the absence of AArch32.
These two are independent features (and in fact our FVP reports/supports both).

Given that, I'm not sure a WARN is really necessary.
Would it be sufficient to just drop the patch for swpX instead?

Thanks!

--
Sincerely,
Yeoreum Yun