linux-kernel - [PATCH v2 26/43] KVM: VMX: Read Posted Interrupt "control" exactly once per loop iteration

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Date:   Fri,  8 Oct 2021 19:12:19 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: [PATCH v2 26/43] KVM: VMX: Read Posted Interrupt "control" exactly
 once per loop iteration

Use READ_ONCE() when loading the posted interrupt descriptor control
field to ensure "old" and "new" have the same base value.  If the
compiler emits separate loads, and loads into "new" before "old", KVM
could theoretically drop the ON bit if it were set between the loads.

Fixes: 28b835d60fcc ("KVM: Update Posted-Interrupts Descriptor when vCPU is preempted")
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/posted_intr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 414ea6972b5c..fea343dcc011 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -53,7 +53,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 
 	/* The full case.  */
 	do {
-		old.control = new.control = pi_desc->control;
+		old.control = new.control = READ_ONCE(pi_desc->control);
 
 		dest = cpu_physical_id(cpu);
 
@@ -104,7 +104,7 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
 	     "Wakeup handler not enabled while the vCPU was blocking");
 
 	do {
-		old.control = new.control = pi_desc->control;
+		old.control = new.control = READ_ONCE(pi_desc->control);
 
 		dest = cpu_physical_id(vcpu->cpu);
 
@@ -160,7 +160,7 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
 	     "Posted Interrupt Suppress Notification set before blocking");
 
 	do {
-		old.control = new.control = pi_desc->control;
+		old.control = new.control = READ_ONCE(pi_desc->control);
 
 		/* set 'NV' to 'wakeup vector' */
 		new.nv = POSTED_INTR_WAKEUP_VECTOR;
-- 
2.33.0.882.g93a45727a2-goog