[PATCH v2 4/5] KVM: nVMX: Deliver missed nested-PI notification-vector via self-IPI while interrupts disabled

[Liran Alon <liran.alon@oracle.com>]

When L1 wants to send a posted-interrupt to another L1 CPU running L2,
it sets the relevant bit in vmx->nested.pi_desc->pir and ON bit in
vmx->nested.pi_desc->control. Then it attempts to send a
notification-vector IPI to dest L1 CPU.
This attempt to send IPI will exit to L0 which will reach
vmx_deliver_nested_posted_interrupt() which does the following:
1. If dest L0 CPU is currently running guest (vcpu->mode ==
IN_GUEST_MODE), it sends a physical IPI of PI nested-notification-vector.
2. It sets KVM_REQ_EVENT in dest vCPU. This is done such that if dest
L0 CPU exits from guest to host and therefore doesn't recognize physical
IPI (or it wasn't sent), then KVM_REQ_EVENT will be consumed on next
vmentry which will call vmx_check_nested_events() which should call
(in theory) vmx_complete_nested_posted_interrupt(). That function
should see vmx->nested.pi_desc->control ON bit is set and therefore
"emulate" posted-interrupt delivery for L1 (sync PIR to IRR in L1
virtual-apic-page & update vmcs02 RVI).

The above logic regarding nested-posted-interrupts contains multiple
issues:

A) Race-condition:
On step (1) it is possible sender will see vcpu->mode == IN_GUEST_MODE
but before sending physical IPI, the dest CPU will exit to host.
Therefore, physical IPI could be received at host which it's hanlder
does nothing. In addition, assume that dest CPU passes the checks
for pending kvm requests before sender sets KVM_REQ_EVENT. Therefore,
dest CPU will resume L2 without evaluating nested-posted-interrupts.

B) vmx_complete_nested_posted_interrupt() is not always called
when needed. Even if dest CPU consumed KVM_REQ_EVENT, there is a bug
that vmx_check_nested_events() could exit from L2 to L1 before calling
vmx_complete_nested_posted_interrupt(). Therefore, on next resume of
L1 into L2, nested-posted-interrupts won't be evaluated even though L0
resume L2 (We may resume L2 without having KVM_REQ_EVENT set).

This commit removes nested-posted-interrupts processing from
check_nested_events() and instead makes sure to process
nested-posted-interrupts on vmentry after interrupts
disabled. Processing of nested-posted-interrupts is delegated
to hardware by issueing a self-IPI of relevant notification-vector
which will be delivered to CPU when CPU is in guest.

* Bug (A) is solved by the fact that processing of
nested-posted-interrupts is not dependent on KVM_REQ_EVENT and
happens before every vmentry to L2.
* Bug (B) is now trivially solved by processing
nested-posted-interrupts before each vmentry to L2 guest.

An alternative could have been to just call
vmx_complete_nested_posted_interrupt() at this call-site. However, we
have decided to go with this approach because:
1. It would require modifying vmx_complete_nested_posted_interrupt()
to be able to work with interrupts disabled (not-trivial).
2. We preffer to avoid software-emulation of hardware behavior if it
is possible.

Fixes: 705699a13994 ("KVM: nVMX: Enable nested posted interrupt processing")

Signed-off-by: Liran Alon <liran.alon@oracle.com>
Co-authored-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/vmx.c              | 57 ++++++++++-------------------------------
 arch/x86/kvm/x86.c              | 14 +++++++---
 3 files changed, 25 insertions(+), 47 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 1bfb99770c34..dc0affe69903 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -980,6 +980,7 @@ struct kvm_x86_ops {
 	void (*set_virtual_x2apic_mode)(struct kvm_vcpu *vcpu, bool set);
 	void (*set_apic_access_page_addr)(struct kvm_vcpu *vcpu, hpa_t hpa);
 	void (*deliver_posted_interrupt)(struct kvm_vcpu *vcpu, int vector);
+	void (*complete_nested_posted_interrupt)(struct kvm_vcpu *vcpu);
 	int (*sync_pir_to_irr)(struct kvm_vcpu *vcpu);
 	int (*set_tss_addr)(struct kvm *kvm, unsigned int addr);
 	int (*get_tdp_level)(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index bbc023fb9ef1..517822f94158 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -534,12 +534,6 @@ static bool pi_test_and_set_on(struct pi_desc *pi_desc)
 			(unsigned long *)&pi_desc->control);
 }
 
-static bool pi_test_and_clear_on(struct pi_desc *pi_desc)
-{
-	return test_and_clear_bit(POSTED_INTR_ON,
-			(unsigned long *)&pi_desc->control);
-}
-
 static int pi_test_and_set_pir(int vector, struct pi_desc *pi_desc)
 {
 	return test_and_set_bit(vector, (unsigned long *)pi_desc->pir);
@@ -5041,37 +5035,6 @@ static void nested_mark_vmcs12_pages_dirty(struct kvm_vcpu *vcpu)
 	}
 }
 
-
-static void vmx_complete_nested_posted_interrupt(struct kvm_vcpu *vcpu)
-{
-	struct vcpu_vmx *vmx = to_vmx(vcpu);
-	int max_irr;
-	void *vapic_page;
-	u16 status;
-
-	if (!vmx->nested.pi_desc)
-		return;
-
-	if (!pi_test_and_clear_on(vmx->nested.pi_desc))
-		return;
-
-	max_irr = find_last_bit((unsigned long *)vmx->nested.pi_desc->pir, 256);
-	if (max_irr != 256) {
-		vapic_page = kmap(vmx->nested.virtual_apic_page);
-		__kvm_apic_update_irr(vmx->nested.pi_desc->pir, vapic_page);
-		kunmap(vmx->nested.virtual_apic_page);
-
-		status = vmcs_read16(GUEST_INTR_STATUS);
-		if ((u8)max_irr > ((u8)status & 0xff)) {
-			status &= ~0xff;
-			status |= (u8)max_irr;
-			vmcs_write16(GUEST_INTR_STATUS, status);
-		}
-	}
-
-	nested_mark_vmcs12_pages_dirty(vcpu);
-}
-
 static inline bool kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,
 						     bool nested)
 {
@@ -5120,11 +5083,6 @@ static int vmx_deliver_nested_posted_interrupt(struct kvm_vcpu *vcpu,
 	    vector == vmx->nested.posted_intr_nv) {
 		/* the PIR and ON have been set by L1. */
 		kvm_vcpu_trigger_posted_interrupt(vcpu, true);
-		/*
-		 * If a posted intr is not recognized by hardware,
-		 * we will accomplish it in the next vmentry.
-		 */
-		kvm_make_request(KVM_REQ_EVENT, vcpu);
 		return 0;
 	}
 	return -1;
@@ -5156,6 +5114,17 @@ static void vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu, int vector)
 		kvm_vcpu_kick(vcpu);
 }
 
+static void vmx_complete_nested_posted_interrupt(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_vmx *vmx = to_vmx(vcpu);
+
+	WARN_ON(!vcpu->arch.apicv_active);
+
+	if (is_guest_mode(vcpu) && vmx->nested.pi_desc &&
+	    pi_test_on(vmx->nested.pi_desc))
+		kvm_vcpu_trigger_posted_interrupt(vcpu, true);
+}
+
 /*
  * Set up the vmcs's constant host-state fields, i.e., host-state fields that
  * will not change in the lifetime of the guest.
@@ -6823,6 +6792,7 @@ static __init int hardware_setup(void)
 	if (!cpu_has_vmx_apicv()) {
 		enable_apicv = 0;
 		kvm_x86_ops->sync_pir_to_irr = NULL;
+		kvm_x86_ops->complete_nested_posted_interrupt = NULL;
 	}
 
 	if (cpu_has_vmx_tsc_scaling()) {
@@ -11144,7 +11114,6 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
 		return 0;
 	}
 
-	vmx_complete_nested_posted_interrupt(vcpu);
 	return 0;
 }
 
@@ -12136,6 +12105,8 @@ static int enable_smi_window(struct kvm_vcpu *vcpu)
 	.hwapic_isr_update = vmx_hwapic_isr_update,
 	.sync_pir_to_irr = vmx_sync_pir_to_irr,
 	.deliver_posted_interrupt = vmx_deliver_posted_interrupt,
+	.complete_nested_posted_interrupt =
+		vmx_complete_nested_posted_interrupt,
 
 	.set_tss_addr = vmx_set_tss_addr,
 	.get_tdp_level = get_ept_level,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 34c85aa2e2d1..6fb58ab9a85c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6962,12 +6962,18 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	smp_mb__after_srcu_read_unlock();
 
 	/*
-	 * This handles the case where a posted interrupt was
-	 * notified with kvm_vcpu_kick.
+	 * In case guest got the posted-interrupt notification
+	 * vector while running in host, we need to make sure
+	 * it arrives to guest.
+	 * For L1 posted-interrupts, we manually sync PIR to IRR.
+	 * For L2 posted-interrupts, we send notification-vector
+	 * again by self IPI such that it will now be received in guest.
 	 */
-	if (kvm_lapic_enabled(vcpu)) {
-		if (kvm_x86_ops->sync_pir_to_irr && vcpu->arch.apicv_active)
+	if (kvm_lapic_enabled(vcpu) && vcpu->arch.apicv_active) {
+		if (kvm_x86_ops->sync_pir_to_irr)
 			kvm_x86_ops->sync_pir_to_irr(vcpu);
+		if (kvm_x86_ops->complete_nested_posted_interrupt)
+			kvm_x86_ops->complete_nested_posted_interrupt(vcpu);
 	}
 
 	if (vcpu->mode == EXITING_GUEST_MODE || kvm_request_pending(vcpu)
-- 
1.9.1