linux-kernel - [PATCH v7 18/31] x86/speculation/mds: Mark cpu_buf_idle

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Message-ID: <20251114150133.1056710-19-vschneid@redhat.com>
Date: Fri, 14 Nov 2025 16:01:20 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	rcu@...r.kernel.org,
	x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	loongarch@...ts.linux.dev,
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	linux-arch@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
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	Andrew Morton <akpm@...ux-foundation.org>,
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	Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: [PATCH v7 18/31] x86/speculation/mds: Mark cpu_buf_idle_clear key as allowed in .noinstr

Later commits will cause objtool to warn about static keys being used in
.noinstr sections in order to safely defer instruction patching IPIs
targeted at NOHZ_FULL CPUs.

cpu_buf_idle_clear is used in .noinstr code, and can be modified at
runtime (SMT hotplug). Suppressing the text_poke_sync() IPI has little
benefits for this key, as hotplug implies eventually going through
takedown_cpu() -> stop_machine_cpuslocked() which is going to cause
interference on all online CPUs anyway.

Mark it to let objtool know not to warn about it.

Signed-off-by: Valentin Schneider <vschneid@...hat.com>
---
 arch/x86/kernel/cpu/bugs.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index a2d7dc4d2a4a3..39f6a2f9593f8 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -181,8 +181,13 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 DEFINE_STATIC_KEY_FALSE(switch_vcpu_ibpb);
 EXPORT_SYMBOL_GPL(switch_vcpu_ibpb);
 
-/* Control CPU buffer clear before idling (halt, mwait) */
-DEFINE_STATIC_KEY_FALSE(cpu_buf_idle_clear);
+/*
+ * Control CPU buffer clear before idling (halt, mwait)
+ *
+ * NOINSTR: This static key is updated during SMT hotplug which itself already
+ * causes some interference.
+ */
+DEFINE_STATIC_KEY_FALSE_NOINSTR(cpu_buf_idle_clear);
 EXPORT_SYMBOL_GPL(cpu_buf_idle_clear);
 
 /*
-- 
2.51.0