linux-kernel - [PATCH 6.1.y 07/21] KVM: x86: Snapshot the host's DEBUGCTL after disabling IRQs

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Message-ID: <20250815001205.2370711-8-seanjc@google.com>
Date: Thu, 14 Aug 2025 17:11:51 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: stable@...r.kernel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Sasha Levin <sashal@...nel.org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 6.1.y 07/21] KVM: x86: Snapshot the host's DEBUGCTL after
 disabling IRQs

[ Upstream commit 189ecdb3e112da703ac0699f4ec76aa78122f911 ]

Snapshot the host's DEBUGCTL after disabling IRQs, as perf can toggle
debugctl bits from IRQ context, e.g. when enabling/disabling events via
smp_call_function_single().  Taking the snapshot (long) before IRQs are
disabled could result in KVM effectively clobbering DEBUGCTL due to using
a stale snapshot.

Cc: stable@...r.kernel.org
Reviewed-and-tested-by: Ravi Bangoria <ravi.bangoria@....com>
Link: https://lore.kernel.org/r/20250227222411.3490595-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2178f6bb8e90..0c3908544205 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4742,7 +4742,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
 	/* Save host pkru register if supported */
 	vcpu->arch.host_pkru = read_pkru();
-	vcpu->arch.host_debugctl = get_debugctlmsr();
 
 	/* Apply any externally detected TSC adjustments (due to suspend) */
 	if (unlikely(vcpu->arch.tsc_offset_adjustment)) {
@@ -10851,6 +10850,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		set_debugreg(0, 7);
 	}
 
+	vcpu->arch.host_debugctl = get_debugctlmsr();
+
 	guest_timing_enter_irqoff();
 
 	for (;;) {
-- 
2.51.0.rc1.163.g2494970778-goog