KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 55 insertions, 0 deletions

diff --git a/io_uring/wait.h b/io_uring/wait.h
new file mode 100644
index 000000000..b7b9c46b1
--- /dev/null
+++ b/io_uring/wait.h
@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef IOU_WAIT_H
+#define IOU_WAIT_H
+
+#include <linux/io_uring_types.h>
+
+/*
+ * ->cq_wait_nr is armed with the number of lazy task_work adds the waiter
+ * still needs, and counted down by the add side, with the add reaching zero
+ * issuing the (single) wake up for this wait cycle. Zero and below means no
+ * wake up is to be issued: IO_CQ_WAKE_INIT when no task is waiting (also
+ * what a forced wake up resets it to when claiming one), zero once the
+ * countdown has fired.
+ */
+#define IO_CQ_WAKE_INIT		(-1)
+
+struct ext_arg {
+	size_t argsz;
+	struct timespec64 ts;
+	const sigset_t __user *sig;
+	ktime_t min_time;
+	bool ts_set;
+	bool iowait;
+};
+
+int io_cqring_wait(struct io_ring_ctx *ctx, int min_events, u32 flags,
+		   struct ext_arg *ext_arg);
+int io_run_task_work_sig(struct io_ring_ctx *ctx);
+void io_cqring_do_overflow_flush(struct io_ring_ctx *ctx);
+void io_cqring_overflow_flush_locked(struct io_ring_ctx *ctx);
+
+static inline unsigned int __io_cqring_events(struct io_ring_ctx *ctx)
+{
+	struct io_rings *rings = io_get_rings(ctx);
+	return ctx->cached_cq_tail - READ_ONCE(rings->cq.head);
+}
+
+static inline unsigned int __io_cqring_events_user(struct io_ring_ctx *ctx)
+{
+	struct io_rings *rings = io_get_rings(ctx);
+
+	return READ_ONCE(rings->cq.tail) - READ_ONCE(rings->cq.head);
+}
+
+/*
+ * Reads the tail/head of the CQ ring while providing an acquire ordering,
+ * see comment at top of io_uring.c.
+ */
+static inline unsigned io_cqring_events(struct io_ring_ctx *ctx)
+{
+	smp_rmb();
+	return __io_cqring_events(ctx);
+}
+
+#endif