Claude review: drm/qxl: Convert qxl release idr to xarray

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Bug: `GFP_NOWAIT` should be `GFP_KERNEL`**

The original code used the IDR preload pattern to ensure allocation success:

```c
idr_preload(GFP_KERNEL);
spin_lock(&qdev->release_idr_lock);
handle = idr_alloc(&qdev->release_idr, release, 1, 0, GFP_NOWAIT);
...
spin_unlock(&qdev->release_idr_lock);
idr_preload_end();
```

`idr_preload(GFP_KERNEL)` pre-allocates memory (sleeping if necessary) so that the subsequent `GFP_NOWAIT` allocation under the spinlock is guaranteed to succeed. The replacement:

```c
r = xa_alloc(&qdev->release_xa, &handle, release, xa_limit_32b, GFP_NOWAIT);
```

uses `GFP_NOWAIT` without any preloading. Since `xa_alloc` may need to allocate internal xarray nodes, and `GFP_NOWAIT` prevents sleeping, this will fail more readily under memory pressure than the original code. Now that no external spinlock is held, `GFP_KERNEL` should be used instead.

**Unnecessary locking around `xa_load`**

In `qxl_release_from_id_locked`:

```c
xa_lock(&qdev->release_xa);
release = xa_load(&qdev->release_xa, id);
xa_unlock(&qdev->release_xa);
```

`xa_load` uses `rcu_read_lock()` internally and is safe to call without holding the xa_lock. The xa_lock is only needed for store operations. This adds unnecessary spinlock contention. A plain `xa_load()` call without the surrounding lock/unlock is sufficient and correct.

**Seqno atomicity — correct but worth noting**

The change from `++qdev->release_seqno` (under spinlock) to `atomic_inc_return(&qdev->release_seqno)` (after xa_alloc returns) introduces a theoretical window where two concurrent allocations could get handles in one order but seqnos in the reverse order. However, since `dma_fence_init` uses `release->id | 0xf0000000` as the fence context (unique per release), cross-release seqno ordering doesn't matter. This is fine.

**Minor: seqno incremented on allocation failure**

```c
r = xa_alloc(&qdev->release_xa, &handle, release, xa_limit_32b, GFP_NOWAIT);
release->base.seqno = atomic_inc_return(&qdev->release_seqno);
if (r < 0) {
```

The seqno is bumped before the error check, wasting a sequence number on failure. The original code had the same behavior, so this isn't a regression, but while touching this code it would be easy to move the seqno assignment after the error check.

**Missing `xa_destroy` in teardown**

`qxl_device_fini()` doesn't call `xa_destroy()` for the new xarray. The original code also omitted `idr_destroy()`, so this isn't a regression, but it would be good to clean up here while converting.

**Variable renames — fine**

The `idr_ret` → `xa_ret` renames in `qxl_alloc_surface_release_reserved` and `qxl_alloc_release_reserved` are straightforward and correct. The error message update from `release_idr` to `release_xa` is appropriate.

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