In commit 14243b3871 ("KVM: x86/xen: Add KVM_IRQ_ROUTING_XEN_EVTCHN
and event channel delivery") the clever version of me left some helpful
notes for those who would come after him:
/*
* For the irqfd workqueue, using the main kvm->lock mutex is
* fine since this function is invoked from kvm_set_irq() with
* no other lock held, no srcu. In future if it will be called
* directly from a vCPU thread (e.g. on hypercall for an IPI)
* then it may need to switch to using a leaf-node mutex for
* serializing the shared_info mapping.
*/
mutex_lock(&kvm->lock);
In commit 2fd6df2f2b ("KVM: x86/xen: intercept EVTCHNOP_send from guests")
the other version of me ran straight past that comment without reading it,
and introduced a potential deadlock by taking vcpu->mutex and kvm->lock
in the wrong order.
Solve this as originally suggested, by adding a leaf-node lock in the Xen
state rather than using kvm->lock for it.
Fixes: 2fd6df2f2b ("KVM: x86/xen: intercept EVTCHNOP_send from guests")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20230111180651.14394-4-dwmw2@infradead.org>
[Rebase, add docs. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
310bc39546