[PATCH] KVM: x86/xen: Convert evtchn_ports from IDR to XArray

[PATCH] KVM: x86/xen: Convert evtchn_ports from IDR to XArray

[Furkan Caliskan]

IDR is deprecated in favor of XArray: see
Documentation/core-api/idr.rst. Convert evtchn_ports accordingly.

kvm_xen_eventfd_assign()'s single-slot idr_alloc() becomes
xa_insert(), since it was really an insert-at-index, not an
allocation: -EBUSY replaces -ENOSPC, still mapped to -EEXIST.

kvm_xen_hcall_evtchn_send() drops its explicit rcu_read_lock(),
since xa_load() takes its own RCU read-side section internally.
evtchnfd's lifetime is still guaranteed by kvm->srcu.

xen_lock is left in place: it protects state beyond the map itself.

Signed-off-by: Furkan Caliskan <frn1furkan10@gmail.com>
---
 arch/x86/include/asm/kvm_host.h |  3 ++-
 arch/x86/kvm/xen.c              | 34 ++++++++++++++++-----------------
 2 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d8700eb848b4..6c8542b32313 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -16,6 +16,7 @@
 #include <linux/irq_work.h>
 #include <linux/irq.h>
 #include <linux/workqueue.h>
+#include <linux/xarray.h>
 
 #include <linux/kvm.h>
 #include <linux/kvm_para.h>
@@ -1290,7 +1291,7 @@ struct kvm_xen {
 	bool runstate_update_flag;
 	u8 upcall_vector;
 	struct gfn_to_pfn_cache shinfo_cache;
-	struct idr evtchn_ports;
+	struct xarray evtchn_ports;
 	unsigned long poll_mask[BITS_TO_LONGS(KVM_MAX_VCPUS)];
 
 	struct kvm_xen_hvm_config hvm_config;
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 694b31c1fcc9..754191e3bef7 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -2072,7 +2072,7 @@ static int kvm_xen_eventfd_update(struct kvm *kvm,
 
 	/* Protect writes to evtchnfd as well as the idr lookup.  */
 	mutex_lock(&kvm->arch.xen.xen_lock);
-	evtchnfd = idr_find(&kvm->arch.xen.evtchn_ports, port);
+	evtchnfd = xa_load(&kvm->arch.xen.evtchn_ports, port);
 
 	ret = -ENOENT;
 	if (!evtchnfd)
@@ -2166,13 +2166,13 @@ static int kvm_xen_eventfd_assign(struct kvm *kvm,
 	}
 
 	mutex_lock(&kvm->arch.xen.xen_lock);
-	ret = idr_alloc(&kvm->arch.xen.evtchn_ports, evtchnfd, port, port + 1,
+	ret = xa_insert(&kvm->arch.xen.evtchn_ports, port, evtchnfd,
 			GFP_KERNEL);
 	mutex_unlock(&kvm->arch.xen.xen_lock);
-	if (ret >= 0)
+	if (!ret)
 		return 0;
 
-	if (ret == -ENOSPC)
+	if (ret == -EBUSY)
 		ret = -EEXIST;
 out:
 	if (eventfd)
@@ -2187,7 +2187,7 @@ static int kvm_xen_eventfd_deassign(struct kvm *kvm, u32 port)
 	struct evtchnfd *evtchnfd;
 
 	mutex_lock(&kvm->arch.xen.xen_lock);
-	evtchnfd = idr_remove(&kvm->arch.xen.evtchn_ports, port);
+	evtchnfd = xa_erase(&kvm->arch.xen.evtchn_ports, port);
 	mutex_unlock(&kvm->arch.xen.xen_lock);
 
 	if (!evtchnfd)
@@ -2203,7 +2203,7 @@ static int kvm_xen_eventfd_deassign(struct kvm *kvm, u32 port)
 static int kvm_xen_eventfd_reset(struct kvm *kvm)
 {
 	struct evtchnfd *evtchnfd, **all_evtchnfds;
-	int i;
+	unsigned long i;
 	int n = 0;
 
 	mutex_lock(&kvm->arch.xen.xen_lock);
@@ -2213,7 +2213,7 @@ static int kvm_xen_eventfd_reset(struct kvm *kvm)
 	 * critical section, first collect all the evtchnfd objects
 	 * in an array as they are removed from evtchn_ports.
 	 */
-	idr_for_each_entry(&kvm->arch.xen.evtchn_ports, evtchnfd, i)
+	xa_for_each(&kvm->arch.xen.evtchn_ports, i, evtchnfd)
 		n++;
 
 	all_evtchnfds = kmalloc_objs(struct evtchnfd *, n);
@@ -2223,9 +2223,9 @@ static int kvm_xen_eventfd_reset(struct kvm *kvm)
 	}
 
 	n = 0;
-	idr_for_each_entry(&kvm->arch.xen.evtchn_ports, evtchnfd, i) {
+	xa_for_each(&kvm->arch.xen.evtchn_ports, i, evtchnfd) {
 		all_evtchnfds[n++] = evtchnfd;
-		idr_remove(&kvm->arch.xen.evtchn_ports, evtchnfd->send_port);
+		xa_erase(&kvm->arch.xen.evtchn_ports, evtchnfd->send_port);
 	}
 	mutex_unlock(&kvm->arch.xen.xen_lock);
 
@@ -2276,12 +2276,10 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r)
 	}
 
 	/*
-	 * evtchnfd is protected by kvm->srcu; the idr lookup instead
-	 * is protected by RCU.
+	 * evtchnfd is protected by kvm->srcu; the xa_load is RCU-safe
+	 * internally, no explicit rcu_read_lock() needed.
 	 */
-	rcu_read_lock();
-	evtchnfd = idr_find(&vcpu->kvm->arch.xen.evtchn_ports, send.port);
-	rcu_read_unlock();
+	evtchnfd = xa_load(&vcpu->kvm->arch.xen.evtchn_ports, send.port);
 	if (!evtchnfd)
 		return false;
 
@@ -2328,23 +2326,23 @@ void kvm_xen_destroy_vcpu(struct kvm_vcpu *vcpu)
 void kvm_xen_init_vm(struct kvm *kvm)
 {
 	mutex_init(&kvm->arch.xen.xen_lock);
-	idr_init(&kvm->arch.xen.evtchn_ports);
+	xa_init(&kvm->arch.xen.evtchn_ports);
 	kvm_gpc_init(&kvm->arch.xen.shinfo_cache, kvm);
 }
 
 void kvm_xen_destroy_vm(struct kvm *kvm)
 {
 	struct evtchnfd *evtchnfd;
-	int i;
+	unsigned long i;
 
 	kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);
 
-	idr_for_each_entry(&kvm->arch.xen.evtchn_ports, evtchnfd, i) {
+	xa_for_each(&kvm->arch.xen.evtchn_ports, i, evtchnfd) {
 		if (!evtchnfd->deliver.port.port)
 			eventfd_ctx_put(evtchnfd->deliver.eventfd.ctx);
 		kfree(evtchnfd);
 	}
-	idr_destroy(&kvm->arch.xen.evtchn_ports);
+	xa_destroy(&kvm->arch.xen.evtchn_ports);
 
 	if (kvm->arch.xen.hvm_config.msr)
 		static_branch_slow_dec_deferred(&kvm_xen_enabled);
-- 
2.34.1

Re: [PATCH] KVM: x86/xen: Convert evtchn_ports from IDR to XArray

[David Woodhouse]

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On Mon, 2026-07-06 at 11:13 +0300, Furkan Caliskan wrote:
> IDR is deprecated in favor of XArray: see
> Documentation/core-api/idr.rst. Convert evtchn_ports accordingly.
> 
> kvm_xen_eventfd_assign()'s single-slot idr_alloc() becomes
> xa_insert(), since it was really an insert-at-index, not an
> allocation: -EBUSY replaces -ENOSPC, still mapped to -EEXIST.
> 
> kvm_xen_hcall_evtchn_send() drops its explicit rcu_read_lock(),
> since xa_load() takes its own RCU read-side section internally.
> evtchnfd's lifetime is still guaranteed by kvm->srcu.
> 
> xen_lock is left in place: it protects state beyond the map itself.
> 
> Signed-off-by: Furkan Caliskan <frn1furkan10@gmail.com>

Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>

Thanks.

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