[PATCH v3 4/7] KVM: x86/xen: Extract delivery of event to vCPU into a separate helper

[Woodhouse, David]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

Move the marking of an event as pending in the target vCPU's vcpu_info
into a separate helper, __kvm_xen_set_evtchn_fast(), invoked after the
shared_info processing is complete and its RCU read-side critical
section has been exited. This makes both halves easier to read.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
[dwmw2: rebased onto the RCU conversion of the GPC locking; the
 read_trylock() failure path in the original no longer exists. The
 caller's kvm->srcu section now extends across the helper call, since
 kvm_gpc_check() on the vcpu_info cache consults the memslot
 generation and the irqfd path enters holding only irq_srcu; in Sean's
 series that was covered by a guard(srcu) spanning the whole function,
 which this series does not carry.]
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Assisted-by: Claude:claude-mythos-5
---
 arch/x86/kvm/xen.c | 104 ++++++++++++++++++++++++++++-----------------
 1 file changed, 65 insertions(+), 39 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 6bcece2439f5..75fbe88c2102 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1759,6 +1759,67 @@ static void kvm_xen_check_poller(struct kvm_vcpu *vcpu, int port)
 	}
 }
 
+/* Called with kvm->srcu held, as kvm_gpc_check() consults the memslots. */
+static void __kvm_xen_set_evtchn_fast(struct kvm_vcpu *vcpu, int port_word_bit)
+{
+	struct gfn_to_pfn_cache *gpc = &vcpu->arch.xen.vcpu_info_cache;
+	bool kick_vcpu = false;
+
+	/* Now switch to the vCPU's vcpu_info to set the index and pending_sel */
+	rcu_read_lock();
+	if (!kvm_gpc_check(gpc, sizeof(struct vcpu_info))) {
+		/*
+		 * Could not access the vcpu_info. Set the bit in-kernel and
+		 * prod the vCPU to deliver it for itself.
+		 */
+		if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
+			kick_vcpu = true;
+		goto out_unlock;
+	}
+
+	if (IS_ENABLED(CONFIG_64BIT) && vcpu->kvm->arch.xen.long_mode) {
+		struct vcpu_info *vcpu_info = gpc->khva;
+
+		if (!test_and_set_bit(port_word_bit, &vcpu_info->evtchn_pending_sel)) {
+			WRITE_ONCE(vcpu_info->evtchn_upcall_pending, 1);
+			kick_vcpu = true;
+		}
+	} else {
+		struct compat_vcpu_info *vcpu_info = gpc->khva;
+
+		if (!test_and_set_bit(port_word_bit,
+				      (unsigned long *)&vcpu_info->evtchn_pending_sel)) {
+			WRITE_ONCE(vcpu_info->evtchn_upcall_pending, 1);
+			kick_vcpu = true;
+		}
+	}
+
+out_unlock:
+	rcu_read_unlock();
+
+	/*
+	 * Deliver the upcall or kick the vCPU only after dropping the RCU
+	 * read lock. Both paths end up in kvm_vcpu_kick(), and the MSI
+	 * delivery also walks the APIC map and takes APIC locks; none of
+	 * that wants to be nested inside the GPC read-side critical
+	 * section, which must be no longer than the accesses to gpc->khva
+	 * above. Invalidation waits for a grace period, so holding the
+	 * read lock across the kick would extend how long a memory
+	 * invalidation is blocked.
+	 */
+	if (!kick_vcpu)
+		return;
+
+	/* For the per-vCPU lapic vector, deliver it as MSI. */
+	if (vcpu->arch.xen.upcall_vector) {
+		kvm_xen_inject_vcpu_vector(vcpu);
+		return;
+	}
+
+	kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
+	kvm_vcpu_kick(vcpu);
+}
+
 /*
  * The return value from this function is propagated to kvm_set_irq() API,
  * so it returns:
@@ -1775,7 +1836,6 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 	struct kvm_vcpu *vcpu;
 	unsigned long *pending_bits, *mask_bits;
 	int port_word_bit;
-	bool kick_vcpu = false;
 	int vcpu_idx, idx, rc;
 
 	vcpu_idx = READ_ONCE(xe->vcpu_idx);
@@ -1825,49 +1885,15 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 		kvm_xen_check_poller(vcpu, xe->port);
 	} else {
 		rc = 1; /* Delivered to the bitmap in shared_info. */
-		/* Now switch to the vCPU's vcpu_info to set the index and pending_sel */
-		gpc = &vcpu->arch.xen.vcpu_info_cache;
-
-		if (!kvm_gpc_check(gpc, sizeof(struct vcpu_info))) {
-			/*
-			 * Could not access the vcpu_info. Set the bit in-kernel
-			 * and prod the vCPU to deliver it for itself.
-			 */
-			if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
-				kick_vcpu = true;
-			goto out_rcu;
-		}
-
-		if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
-			struct vcpu_info *vcpu_info = gpc->khva;
-			if (!test_and_set_bit(port_word_bit, &vcpu_info->evtchn_pending_sel)) {
-				WRITE_ONCE(vcpu_info->evtchn_upcall_pending, 1);
-				kick_vcpu = true;
-			}
-		} else {
-			struct compat_vcpu_info *vcpu_info = gpc->khva;
-			if (!test_and_set_bit(port_word_bit,
-					      (unsigned long *)&vcpu_info->evtchn_pending_sel)) {
-				WRITE_ONCE(vcpu_info->evtchn_upcall_pending, 1);
-				kick_vcpu = true;
-			}
-		}
-
-		/* For the per-vCPU lapic vector, deliver it as MSI. */
-		if (kick_vcpu && vcpu->arch.xen.upcall_vector) {
-			kvm_xen_inject_vcpu_vector(vcpu);
-			kick_vcpu = false;
-		}
 	}
 
  out_rcu:
 	rcu_read_unlock();
-	srcu_read_unlock(&kvm->srcu, idx);
 
-	if (kick_vcpu) {
-		kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
-		kvm_vcpu_kick(vcpu);
-	}
+	if (rc == 1)
+		__kvm_xen_set_evtchn_fast(vcpu, port_word_bit);
+
+	srcu_read_unlock(&kvm->srcu, idx);
 
 	return rc;
 }
-- 
2.43.0

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