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, 91 insertions, 0 deletions

diff --git a/io_uring/loop.c b/io_uring/loop.c
new file mode 100644
index 000000000..bbbb6ef14
--- /dev/null
+++ b/io_uring/loop.c
@@ -0,0 +1,91 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#include "io_uring.h"
+#include "wait.h"
+#include "loop.h"
+
+static inline int io_loop_nr_cqes(const struct io_ring_ctx *ctx,
+				  const struct iou_loop_params *lp)
+{
+	return lp->cq_wait_idx - READ_ONCE(ctx->rings->cq.tail);
+}
+
+static inline void io_loop_wait_start(struct io_ring_ctx *ctx, unsigned nr_wait)
+{
+	atomic_set(&ctx->cq_wait_nr, nr_wait);
+	set_current_state(TASK_INTERRUPTIBLE);
+}
+
+static inline void io_loop_wait_finish(struct io_ring_ctx *ctx)
+{
+	__set_current_state(TASK_RUNNING);
+	atomic_set(&ctx->cq_wait_nr, IO_CQ_WAKE_INIT);
+}
+
+static void io_loop_wait(struct io_ring_ctx *ctx, struct iou_loop_params *lp,
+			 unsigned nr_wait)
+{
+	io_loop_wait_start(ctx, nr_wait);
+
+	if (unlikely(io_local_work_pending(ctx) ||
+		     io_loop_nr_cqes(ctx, lp) <= 0) ||
+		     READ_ONCE(ctx->check_cq)) {
+		io_loop_wait_finish(ctx);
+		return;
+	}
+
+	mutex_unlock(&ctx->uring_lock);
+	schedule();
+	io_loop_wait_finish(ctx);
+	mutex_lock(&ctx->uring_lock);
+}
+
+static int __io_run_loop(struct io_ring_ctx *ctx)
+{
+	struct iou_loop_params lp = {};
+
+	while (true) {
+		int nr_wait, step_res;
+
+		if (unlikely(!ctx->loop_step))
+			return -EFAULT;
+
+		step_res = ctx->loop_step(io_loop_mangle_ctx(ctx), &lp);
+		if (step_res == IOU_LOOP_STOP)
+			break;
+		if (step_res != IOU_LOOP_CONTINUE)
+			return -EINVAL;
+
+		nr_wait = io_loop_nr_cqes(ctx, &lp);
+		if (nr_wait > 0)
+			io_loop_wait(ctx, &lp, nr_wait);
+		else
+			nr_wait = 0;
+
+		if (task_work_pending(current)) {
+			mutex_unlock(&ctx->uring_lock);
+			io_run_task_work();
+			mutex_lock(&ctx->uring_lock);
+		}
+		if (unlikely(task_sigpending(current)))
+			return -EINTR;
+		io_run_local_work_locked(ctx, nr_wait);
+
+		if (READ_ONCE(ctx->check_cq) & BIT(IO_CHECK_CQ_OVERFLOW_BIT))
+			io_cqring_overflow_flush_locked(ctx);
+	}
+
+	return 0;
+}
+
+int io_run_loop(struct io_ring_ctx *ctx)
+{
+	int ret;
+
+	if (!io_allowed_run_tw(ctx))
+		return -EEXIST;
+
+	mutex_lock(&ctx->uring_lock);
+	ret = __io_run_loop(ctx);
+	mutex_unlock(&ctx->uring_lock);
+	return ret;
+}