linux-kernel - Re: [PATCH v12 2/7] arm64: cpufeature: add FEAT

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Message-ID: <aYsAaaQgBaLbDSsW@e129823.arm.com>
Date: Tue, 10 Feb 2026 09:54:49 +0000
From: Yeoreum Yun <yeoreum.yun@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org, will@...nel.org, maz@...nel.org,
	broonie@...nel.org, oliver.upton@...ux.dev, miko.lenczewski@....com,
	kevin.brodsky@....com, ardb@...nel.org, suzuki.poulose@....com,
	lpieralisi@...nel.org, scott@...amperecomputing.com,
	joey.gouly@....com, yuzenghui@...wei.com, pbonzini@...hat.com,
	shuah@...nel.org, mark.rutland@....com, arnd@...db.de
Subject: Re: [PATCH v12 2/7] arm64: cpufeature: add FEAT_LSUI

Hi Catalin,

> On Fri, Feb 06, 2026 at 06:42:19PM +0000, Catalin Marinas wrote:
> > On Wed, Jan 21, 2026 at 07:06:17PM +0000, Yeoreum Yun wrote:
> > > +#ifdef CONFIG_ARM64_LSUI
> > > +static bool has_lsui(const struct arm64_cpu_capabilities *entry, int scope)
> > > +{
> > > +	if (!has_cpuid_feature(entry, scope))
> > > +		return false;
> > > +
> > > +	/*
> > > +	 * A CPU that supports LSUI should also support FEAT_PAN,
> > > +	 * so that SW_PAN handling is not required.
> > > +	 */
> > > +	if (WARN_ON(!__system_matches_cap(ARM64_HAS_PAN)))
> > > +		return false;
> > > +
> > > +	return true;
> > > +}
> > > +#endif
> >
> > I still find this artificial dependency a bit strange. Maybe one doesn't
> > want any PAN at all (software or hardware) and won't get LSUI either
> > (it's unlikely but possible).
> > We have the uaccess_ttbr0_*() calls already for !LSUI, so maybe
> > structuring the macros in a way that they also take effect with LSUI.
> > For futex, we could add some new functions like uaccess_enable_futex()
> > which wouldn't do anything if LSUI is enabled with hw PAN.
>
> Hmm, I forgot that we removed CONFIG_ARM64_PAN for 7.0, so it makes it
> harder to disable. Give it a try but if the macros too complicated, we
> can live with the additional check in has_lsui().
>
> However, for completeness, we need to check the equivalent of
> !system_uses_ttbr0_pan() but probing early, something like:
>
> 	if (IS_ENABLED(CONFIG_ARM64_SW_TTBR0_PAN) &&
> 	    !__system_matches_cap(ARM64_HAS_PAN)) {
> 		pr_info_once("TTBR0 PAN incompatible with FEAT_LSUI; disabling FEAT_LSUI");
> 		return false;
> 	}
>
> --

TBH, I'm not sure whether it's a artifical dependency or not.
AFAIK, FEAT_PAN is mandatory from Armv8.1 and the FEAT_LSUI seems to
implements based on the present of "FEAT_PAN".

So, for a hardware which doesn't have FEAT_PAN but has FEAT_LSUI
sounds like "wrong" hardware and I'm not sure whether it's right
to enable FEAT_LSUI in this case.

SW_PAN case is the same problem. Since If system uses SW_PAN,
that means this hardware doesn't have a "FEAT_PAN"
So this question seems to ultimately boil down to whether
it is appropriate to allow the use of FEAT_LSUI
even when FEAT_PAN is not supported.

That's why I think the purpose of "has_lsui()" is not for artifical
dependency but to disable for unlike case which have !FEAT_PAN and FEAT_LSUI
and IMHO it's enough to check only check with "ARM64_HAS_PAN" instead of
making a new function like uaccess_enable_futext().

Am I missing something?


Thanks.

--
Sincerely,
Yeoreum Yun