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

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Message-ID: <aYouAv_EjICIN8oA@arm.com>
Date: Mon, 9 Feb 2026 18:57:06 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Yeoreum Yun <yeoreum.yun@....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

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;
	}

-- 
Catalin