linux-kernel - [PATCH v7 20/31] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush

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Message-ID: <20251114151048.1061644-2-vschneid@redhat.com>
Date: Fri, 14 Nov 2025 16:10:47 +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,
	linux-riscv@...ts.infradead.org,
	linux-arch@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Cc: Josh Poimboeuf <jpoimboe@...nel.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
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Subject: [PATCH v7 20/31] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush_cond keys 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.

The VMX keys are used in .noinstr code, and can be modified at runtime
(/proc/kernel/vmx* write). However it is not expected that they will be
flipped during latency-sensitive operations, and thus shouldn't be a source
of interference for NOHZ_FULL CPUs wrt the text patching IPI.

Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
being patched, i.e. this is purely to tell objtool we're okay with updates
to that key causing IPIs and to silence the associated objtool warning.

Reported-by: Josh Poimboeuf <jpoimboe@...nel.org>
Signed-off-by: Valentin Schneider <vschneid@...hat.com>
Acked-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 91b6f2f3edc2a..99936a2af6641 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -203,8 +203,15 @@ module_param(pt_mode, int, S_IRUGO);
 
 struct x86_pmu_lbr __ro_after_init vmx_lbr_caps;
 
-static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
-static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
+/*
+ * NOINSTR: Both of these static keys end up being used in .noinstr sections,
+ * however they are only modified:
+ * - at init
+ * - from a /proc/kernel/vmx* write
+ * thus during latency-sensitive operations they should remain stable.
+ */
+static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_should_flush);
+static DEFINE_STATIC_KEY_FALSE_NOINSTR(vmx_l1d_flush_cond);
 static DEFINE_MUTEX(vmx_l1d_flush_mutex);
 
 /* Storage for pre module init parameter parsing */
-- 
2.51.0