From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
When delivering a virtual interrupt, don't actually send a posted interrupt
if the target vCPU is also the currently running vCPU and is IN_GUEST_MODE,
in which case the interrupt is being sent from a VM-Exit fastpath and the
core run loop in vcpu_enter_guest() will manually move the interrupt from
the PIR to vmcs.GUEST_RVI. IRQs are disabled while IN_GUEST_MODE, thus
there's no possibility of the virtual interrupt being sent from anything
other than KVM, i.e. KVM won't suppress a wake event from an IRQ handler
(see commit fdba608f15e2, "KVM: VMX: Wake vCPU when delivering posted IRQ
even if vCPU == this vCPU").
Eliding the posted interrupt restores the performance provided by the
combination of commits 379a3c8ee444 ("KVM: VMX: Optimize posted-interrupt
delivery for timer fastpath") and 26efe2fd92e5 ("KVM: VMX: Handle
preemption timer fastpath").
Thanks Sean for better comments.
Suggested-by: Chao Gao <chao.gao@xxxxxxxxx>
Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
arch/x86/kvm/vmx/vmx.c | 40 +++++++++++++++++++++-------------------
1 file changed, 21 insertions(+), 19 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fe06b02994e6..e06377c9a4cf 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3908,31 +3908,33 @@ static inline void kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,
#ifdef CONFIG_SMP
if (vcpu->mode == IN_GUEST_MODE) {
/*
- * The vector of interrupt to be delivered to vcpu had
- * been set in PIR before this function.
+ * The vector of the virtual has already been set in the PIR.
+ * Send a notification event to deliver the virtual interrupt
+ * unless the vCPU is the currently running vCPU, i.e. the
+ * event is being sent from a fastpath VM-Exit handler, in
+ * which case the PIR will be synced to the vIRR before
+ * re-entering the guest.
*
- * Following cases will be reached in this block, and
- * we always send a notification event in all cases as
- * explained below.
+ * When the target is not the running vCPU, the following
+ * possibilities emerge:
*
- * Case 1: vcpu keeps in non-root mode. Sending a
- * notification event posts the interrupt to vcpu.
+ * Case 1: vCPU stays in non-root mode. Sending a notification
+ * event posts the interrupt to the vCPU.
*
- * Case 2: vcpu exits to root mode and is still
- * runnable. PIR will be synced to vIRR before the
- * next vcpu entry. Sending a notification event in
- * this case has no effect, as vcpu is not in root
- * mode.
+ * Case 2: vCPU exits to root mode and is still runnable. The
+ * PIR will be synced to the vIRR before re-entering the guest.
+ * Sending a notification event is ok as the host IRQ handler
+ * will ignore the spurious event.
*
- * Case 3: vcpu exits to root mode and is blocked.
- * vcpu_block() has already synced PIR to vIRR and
- * never blocks vcpu if vIRR is not cleared. Therefore,
- * a blocked vcpu here does not wait for any requested
- * interrupts in PIR, and sending a notification event
- * which has no effect is safe here.
+ * Case 3: vCPU exits to root mode and is blocked. vcpu_block()
+ * has already synced PIR to vIRR and never blocks the vCPU if
+ * the vIRR is not empty. Therefore, a blocked vCPU here does
+ * not wait for any requested interrupts in PIR, and sending a
+ * notification event also results in a benign, spurious event.
*/
- apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
+ if (vcpu != kvm_get_running_vcpu())
+ apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
return;
}
#endif
--
2.25.1
