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

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Message-ID: <aW9Lrm2+oc4xWv9j@e129823.arm.com>
Date: Tue, 20 Jan 2026 09:32:30 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: Will Deacon <will@...nel.org>
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.

Hi,
> Hi Will,
>
> > 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).
> >
>
> 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.
>
> For that reason, I think it also makes sense to apply LSUI to SWPx emulation.
> That said, since there are currently no real CPUs that actually implement LSUI,
> it is hard to say that this is particularly useful right now.
> I do not have a strong opinion on whether this patch should be applied or
> dropped at this point.
> Personally, given that most CPUs released so far support 32-bit compatibility,
> I expect that future CPUs with LSUI will also retain 32-bit compatibility.
> However, it is difficult to say with certainty which approach
> is correct at this time.
>
> I would therefore like to defer the final decision on this to the maintainers
>
> Am I missing something?

Ah, the Marc view was right. So I think the changes for swpX could be
droppable.

Thanks.

--
Sincerely,
Yeoreum Yun