Claude review: drm/amdgpu: replace PASID IDR with XArray

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

Patch Review

**Critical bug — wrong locking wrappers:**

```c
+	r = xa_alloc_cyclic(&amdgpu_pasid_xa, &pasid, xa_mk_value(0),
+			    XA_LIMIT(1, (1U << bits) - 1),
+			    &amdgpu_pasid_xa_next, GFP_KERNEL);
```

This must be `xa_alloc_cyclic_irq()` to disable interrupts while holding the xa_lock. The plain `xa_alloc_cyclic()` uses `xa_lock()`/`xa_unlock()` (see `include/linux/xarray.h:976-987`), which does not disable IRQs.

```c
+	xa_erase(&amdgpu_pasid_xa, pasid);
```

This must be `xa_erase_irq()`. The plain `xa_erase()` uses `xa_lock()`/`xa_unlock()` (see `lib/xarray.c:1665-1674`), which does not disable IRQs. Since `amdgpu_pasid_free()` can be called from hardirq context (via the fence signal path documented in the commit message), and `xa_alloc_cyclic()` in process context takes the same lock without disabling IRQs, the exact same deadlock scenario described in the commit message still applies.

**The fix:** Replace `xa_alloc_cyclic()` with `xa_alloc_cyclic_irq()` and `xa_erase()` with `xa_erase_irq()`. Alternatively, use explicit `xa_lock_irqsave()`/`__xa_erase()`/`xa_unlock_irqrestore()` in `amdgpu_pasid_free()` (since it's called from hardirq, `_irqsave` is the safest choice there), and `xa_alloc_cyclic_irq()` for the allocation path.

**Note:** The v5 changelog mentions that the author previously used explicit `xa_lock_irqsave`/`__xa_erase` for this exact reason, but was told to use the convenience wrappers in v6. The v6/v7 approach unfortunately reintroduces the problem.

**Minor nits (non-blocking):**

- The `XA_FLAGS_LOCK_IRQ` flag in `DEFINE_XARRAY_FLAGS` is not harmful, but it also isn't doing what the author thinks. It sets the lock class for lockdep — it doesn't make convenience wrappers use IRQ-safe locking. It's still correct to set it (it tells lockdep the lock is used in IRQ context), but the actual API calls must be the `_irq` variants.

- The `xa_mk_value(0)` store is fine — it stores a non-NULL value so the slot is occupied, preventing double-allocation of the same PASID.

- The `XA_FLAGS_ALLOC1` usage is correct (zero is not a valid PASID), though the `XA_LIMIT(1, ...)` already enforces the minimum of 1.

- The cleanup function change to `xa_destroy()` is correct and straightforward.

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