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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
Best regards,
Maxim Levitsky
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