linux-kernel - [PATCH 09/19] arm64: Exclude nohz_full CPUs from 32bits el0 support

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Message-ID: <20241211154035.75565-10-frederic@kernel.org>
Date: Wed, 11 Dec 2024 16:40:22 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 09/19] arm64: Exclude nohz_full CPUs from 32bits el0 support

Nohz full CPUs are not a desirable fallback target to run 32bits el0
applications. If present, prefer a set of housekeeping CPUs that can do
the job instead. Otherwise just don't support el0 32 bits. Should the
need arise, appropriate support can be introduced in the future.

Suggested-by: Will Deacon <will@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 Documentation/arch/arm64/asymmetric-32bit.rst | 9 +++++++++
 arch/arm64/kernel/cpufeature.c                | 6 +++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
index 64a0b505da7d..0c762cfc0f34 100644
--- a/Documentation/arch/arm64/asymmetric-32bit.rst
+++ b/Documentation/arch/arm64/asymmetric-32bit.rst
@@ -153,3 +153,12 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
 mode will return to host userspace with an ``exit_reason`` of
 ``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
 re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
+
+NOHZ FULL
+--------
+
+Nohz full CPUs are not a desirable fallback target to run 32bits el0
+applications. If present, a set of housekeeping CPUs that can do
+the job instead is preferred. Otherwise 32-bit EL0 is not supported.
+Should the need arise, appropriate support can be introduced in the
+future.
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 6ce71f444ed8..7ce1b8ab417f 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -75,6 +75,7 @@
 #include <linux/cpu.h>
 #include <linux/kasan.h>
 #include <linux/percpu.h>
+#include <linux/sched/isolation.h>
 
 #include <asm/cpu.h>
 #include <asm/cpufeature.h>
@@ -3742,7 +3743,10 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
 	static int lucky_winner = -1;
 
 	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
-	bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
+	bool cpu_32bit = false;
+
+	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0) && housekeeping_cpu(cpu, HK_TYPE_TICK))
+		cpu_32bit = true;
 
 	if (cpu_32bit) {
 		cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
-- 
2.46.0