Commit 29802380 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

KVM: VMX: Drop pointless PI.NDST update when blocking 



Don't update Posted Interrupt's NDST, a.k.a. the target pCPU, in the
pre-block path, as NDST is guaranteed to be up-to-date.  The comment
about the vCPU being preempted during the update is simply wrong, as the
update path runs with IRQs disabled (from before snapshotting vcpu->cpu,
until after the update completes).

Since commit 8b306e2f ("KVM: VMX: avoid double list add with VT-d
posted interrupts", 2017-09-27) The vCPU can get preempted _before_
the update starts, but not during.  And if the vCPU is preempted before,
vmx_vcpu_pi_load() is responsible for updating NDST when the vCPU is
scheduled back in.  In that case, the check against the wakeup vector in
vmx_vcpu_pi_load() cannot be true as that would require the notification
vector to have been set to the wakeup vector _before_ blocking.

Opportunistically switch to using vcpu->cpu for the list/lock lookups,
which do not need pre_pcpu since the same commit.

Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-25-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 74ba5bc8

arch/x86/kvm/vmx/posted_intr.c

+3 −20
Original line number Diff line number Diff line
@@ -135,7 +135,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu) 
 * - Store the vCPU to the wakeup list, so when interrupts happen

 *   we can find the right vCPU to wake up.

 * - Change the Posted-interrupt descriptor as below:

 *      'NDST' <-- vcpu->pre_pcpu

 *      'NV' <-- POSTED_INTR_WAKEUP_VECTOR

 * - If 'ON' is set during this process, which means at least one

 *   interrupt is posted for this vCPU, we cannot block it, in
@@ -144,7 +143,6 @@ static void __pi_post_block(struct kvm_vcpu *vcpu) 
 */

int pi_pre_block(struct kvm_vcpu *vcpu)

{

	unsigned int dest;

	struct pi_desc old, new;

	struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
@@ -156,10 +154,10 @@ int pi_pre_block(struct kvm_vcpu *vcpu) 
	local_irq_disable();



	vcpu->pre_pcpu = vcpu->cpu;

	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));

	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));

	list_add_tail(&vcpu->blocked_vcpu_list,

		      &per_cpu(blocked_vcpu_on_cpu, vcpu->pre_pcpu));

	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));

		      &per_cpu(blocked_vcpu_on_cpu, vcpu->cpu));

	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->cpu));



	do {

		old.control = new.control = pi_desc->control;
@@ -168,21 +166,6 @@ int pi_pre_block(struct kvm_vcpu *vcpu) 
		     "Warning: SN field of posted-interrupts "

		     "is set before blocking\n");



		/*

		 * Since vCPU can be preempted during this process,

		 * vcpu->cpu could be different with pre_pcpu, we

		 * need to set pre_pcpu as the destination of wakeup

		 * notification event, then we can find the right vCPU

		 * to wakeup in wakeup handler if interrupts happen

		 * when the vCPU is in blocked state.

		 */

		dest = cpu_physical_id(vcpu->pre_pcpu);



		if (x2apic_mode)

			new.ndst = dest;

		else

			new.ndst = (dest << 8) & 0xFF00;



		/* set 'NV' to 'wakeup vector' */

		new.nv = POSTED_INTR_WAKEUP_VECTOR;

	} while (cmpxchg64(&pi_desc->control, old.control,