KVM: x86: Trigger I/O APIC route rescan in kvm_arch_irq_routing_update()

Trigger the I/O APIC route rescan that's performed for a split IRQ chip
after userspace updates IRQ routes in kvm_arch_irq_routing_update(), i.e.
before dropping kvm->irq_lock.  Calling kvm_make_all_cpus_request() under
a mutex is perfectly safe, and the smp_wmb()+smp_mb__after_atomic() pair
in __kvm_make_request()+kvm_check_request() ensures the new routing is
visible to vCPUs prior to the request being visible to vCPUs.

In all likelihood, commit b053b2aef25d ("KVM: x86: Add EOI exit bitmap
inference") somewhat arbitrarily made the request outside of irq_lock to
avoid holding irq_lock any longer than is strictly necessary.  And then
commit abdb080f7ac8 ("kvm/irqchip: kvm_arch_irq_routing_update renaming
split") took the easy route of adding another arch hook instead of risking
a functional change.

Note, the call to synchronize_srcu_expedited() does NOT provide ordering
guarantees with respect to vCPUs scanning the new routing; as above, the
request infrastructure provides the necessary ordering.  I.e. there's no
need to wait for kvm_scan_ioapic_routes() to complete if it's actively
running, because regardless of whether it grabs the old or new table, the
vCPU will have another KVM_REQ_SCAN_IOAPIC pending, i.e. will rescan again
and see the new mappings.

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 0 insertions, 4 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3bde4fb5c6aa..9461517b4e62 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1024,14 +1024,10 @@ void vcpu_put(struct kvm_vcpu *vcpu);
 
 #ifdef __KVM_HAVE_IOAPIC
 void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm);
-void kvm_arch_post_irq_routing_update(struct kvm *kvm);
 #else
 static inline void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm)
 {
 }
-static inline void kvm_arch_post_irq_routing_update(struct kvm *kvm)
-{
-}
 #endif
 
 #ifdef CONFIG_HAVE_KVM_IRQCHIP