linux-kernel - [PATCH] arm64/fpsimd: Ensure that offlined CPUs are not using SME

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Message-Id: <20240618-arm64-fpsimd-sme-cpu-die-v1-1-9a90d1a34918@kernel.org>
Date: Tue, 18 Jun 2024 15:03:50 +0100
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>, 
 Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Mark Brown <broonie@...nel.org>
Subject: [PATCH] arm64/fpsimd: Ensure that offlined CPUs are not using SME

When we use CPU hotplug to offline a CPU we may transition directly from
running a task which was using SME to the CPU being offlined. This means
that PSTATE.{SM,ZA} may still be set, indicating to the system that SME is
still in use. This could create contention with other still running CPUs if
the system uses shared SMCUs.

For most systems this shouldn't be an issue, we should have PSCI or some
other power management mechanism which will take care of this as part of
offlining the CPU. However we do still have support for spin tables, and it
is possible that system firmware may not be ideally implemented, so let's
explicitly disable SME during the process of offlining the CPU in order to
ensure there's no spurious contention.

Signed-off-by: Mark Brown <broonie@...nel.org>
---
 arch/arm64/kernel/smp.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 31c8b3094dd7..9e8fc6ac758a 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -383,6 +383,10 @@ void __noreturn cpu_die(void)
 	/* Tell cpuhp_bp_sync_dead() that this CPU is now safe to dispose of */
 	cpuhp_ap_report_dead();

+	/* Ensure we are not spuriously contending any SMCU */
+	if (system_supports_sme())
+		sme_smstop();
+
 	/*
 	 * Actually shutdown the CPU. This must never fail. The specific hotplug
 	 * mechanism must perform all required cache maintenance to ensure that

---
base-commit: 83a7eefedc9b56fe7bfeff13b6c7356688ffa670
change-id: 20240617-arm64-fpsimd-sme-cpu-die-57205c7f220e

Best regards,
-- 
Mark Brown <broonie@...nel.org>