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

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Message-ID: <aQ3/35146HkAqM+I@e129823.arm.com>
Date: Fri, 7 Nov 2025 14:19:11 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>, miko.lenczewski@....com,
	Kevin Brodsky <kevin.brodsky@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>, yangyicong@...ilicon.com,
	scott@...amperecomputing.com, Joey Gouly <joey.gouly@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Paolo Bonzini <pbonzini@...hat.com>, shuah <shuah@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	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 9/9] arm64: armv8_deprecated: apply FEAT_LSUI for
 swpX emulation.

Hi Arnd.

> On Thu, Nov 6, 2025, at 10:40, Yeoreum Yun wrote:
> > apply FEAT_LSUI instruction to emulate deprecated swpX instruction.
>
> Can you explain in the changrelog why you do this?
>
> In particular, is this a performance optimization or is this required
> for correctness in some scenario?

I think the main purpose using for this is to "remove the toggling the
PSTATE.PAN" bit. so that remove some *potential* problem which can
happen when PAN bit is cleared.

also, though swpb might add some complexity, but swp can get
a little bit of benifit by removing the ll/sc way and reduce
the amount of instructions.

> I would have expected that there is very little benefit in
> changing the swp/swpb emulation here if the existing code
> has to remain compiled into the kernel and the overhead of
> the trap is so much larger than the uaccess_enable_privileged()
> overhead.

Yeap. since the *trap* overhead is much larger than the
uaccess_enable_privileged() in the performnace perspecitve,
It would has a little effect. However, as I mention above,
the main purpose of this is to remove toggling the PSTATE.PAN
if possible. there would be a little bit of additional commplexity
for swpb but it seems affordable.

Thanks.

[...]

--
Sincerely,
Yeoreum Yun