KVM: x86/xen: fix recursive deadlock in timer injection

The fast-path timer delivery introduced a recursive locking deadlock
when userspace configures a timer which has already expired and is
delivered immediately. The call to kvm_xen_inject_timer_irqs() can
call to kvm_xen_set_evtchn() which may take kvm->arch.xen.xen_lock,
which is already held in kvm_xen_vcpu_get_attr().

 ============================================
 WARNING: possible recursive locking detected
 6.8.0-smp--5e10b4d51d77-drs torvalds#232 Tainted: G           O
 --------------------------------------------
 xen_shinfo_test/250013 is trying to acquire lock:
 ffff938c9930cc30 (&kvm->arch.xen.xen_lock){+.+.}-{3:3}, at: kvm_xen_set_evtchn+0x74/0x170 [kvm]

 but task is already holding lock:
 ffff938c9930cc30 (&kvm->arch.xen.xen_lock){+.+.}-{3:3}, at: kvm_xen_vcpu_get_attr+0x38/0x250 [kvm]

Now that the gfn_to_pfn_cache has its own self-sufficient locking, its
callers no longer need to ensure serialization, so just stop taking
kvm->arch.xen.xen_lock from kvm_xen_set_evtchn().

Fixes: 77c9b9d ("KVM: x86/xen: Use fast path for Xen timer delivery")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240227115648.3104-6-dwmw2@infradead.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/xen.c

@@ -1862,8 +1862,6 @@ static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 		mm_borrowed = true;
 	}
 
-	mutex_lock(&kvm->arch.xen.xen_lock);
-
 	/*
 	 * It is theoretically possible for the page to be unmapped
 	 * and the MMU notifier to invalidate the shared_info before
@@ -1891,8 +1889,6 @@ static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 		srcu_read_unlock(&kvm->srcu, idx);
 	} while(!rc);
 
-	mutex_unlock(&kvm->arch.xen.xen_lock);
-
 	if (mm_borrowed)
 		kthread_unuse_mm(kvm->mm);