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    SubjectRe: [PATCH v2 26/43] KVM: VMX: Read Posted Interrupt "control" exactly once per loop iteration
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    On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
    > 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@google.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;

    I wish there was a way to mark fields in a struct, as requiring 'READ_ONCE' on them
    so that compiler would complain if this isn't done, or automatically use 'READ_ONCE'
    logic.

    Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

    Best regards,
    Maxim Levitsky

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