linux-kernel - [RFC PATCH v2 1/8] arm64: kpkeys: Avoid unnecessary writes to POR

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Message-ID: <20250815090000.2182450-2-kevin.brodsky@arm.com>
Date: Fri, 15 Aug 2025 09:59:53 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: linux-hardening@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	Kevin Brodsky <kevin.brodsky@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	David Howells <dhowells@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jann Horn <jannh@...gle.com>,
	Jeff Xu <jeffxu@...omium.org>,
	Joey Gouly <joey.gouly@....com>,
	Kees Cook <kees@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Marc Zyngier <maz@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Maxwell Bland <mbland@...orola.com>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Pierre Langlois <pierre.langlois@....com>,
	Quentin Perret <qperret@...gle.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-mm@...ck.org,
	x86@...nel.org
Subject: [RFC PATCH v2 1/8] arm64: kpkeys: Avoid unnecessary writes to POR_EL1

Nested uses of kpkeys guards are about to be introduced, which means
that kpkeys_set_level() may not actually need to change the value of
POR_EL1. Since updating POR_EL1 requires an expensive ISB, let's
skip the write if the value is unchanged, by returning
KPKEYS_PKEY_REG_INVAL. This will cause the matching
kpkeys_restore_pkey_reg() call to bail out without calling
arch_kpkeys_restore_pkey_reg().

Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
---
 arch/arm64/include/asm/kpkeys.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm64/include/asm/kpkeys.h b/arch/arm64/include/asm/kpkeys.h
index 64d6e22740ec..70e21df84252 100644
--- a/arch/arm64/include/asm/kpkeys.h
+++ b/arch/arm64/include/asm/kpkeys.h
@@ -43,6 +43,9 @@ static __always_inline int arch_kpkeys_set_level(int level)
 	u64 prev_por = read_sysreg_s(SYS_POR_EL1);
 	u64 new_por = por_set_kpkeys_level(prev_por, level);
 
+	if (new_por == prev_por)
+		return KPKEYS_PKEY_REG_INVAL;
+
 	__kpkeys_set_pkey_reg_nosync(new_por);
 	isb();
 
-- 
2.47.0