asahi: bind zero-page

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33682>

src/asahi/lib/agx_abi.h

@@ -19,3 +19,9 @@
 #define AGX_ABI_FOUT_S             (6)
 #define AGX_ABI_FOUT_WRITE_SAMPLES (7)
 #define AGX_ABI_FOUT_COLOUR(rt)    (2 * (4 + (4 * rt)))
+
+/* This address is in our reservation, and can be
+ * addressed with only small integers in the low/high. That lets us do some
+ * robustness optimization even without soft fault.
+ */
+#define AGX_ZERO_PAGE_ADDRESS (1ull << 32)

src/asahi/lib/agx_device.c

@@ -11,6 +11,7 @@
 #include "util/macros.h"
 #include "util/ralloc.h"
 #include "util/timespec.h"
+#include "agx_abi.h"
 #include "agx_bo.h"
 #include "agx_compile.h"
 #include "agx_device_virtio.h"
@@ -680,6 +681,20 @@ agx_open_device(void *memctx, struct agx_device *dev)
       dev->chip = AGX_CHIP_G13G;
    }
 
+   /* Bind read-only zero page at 2^32. This is in our reservation, and can be
+    * addressed with only small integers in the low/high. That lets us do some
+    * robustness optimization even without soft fault.
+    */
+   {
+      void *bo = agx_bo_create(dev, 16384, 0, 0, "Zero page");
+      int ret = dev->ops.bo_bind(dev, bo, AGX_ZERO_PAGE_ADDRESS, 16384, 0,
+                                 ASAHI_BIND_READ, false);
+      if (ret) {
+         fprintf(stderr, "Failed to bind zero page");
+         return false;
+      }
+   }
+
    void *bo = agx_bo_create(dev, LIBAGX_PRINTF_BUFFER_SIZE, 0, AGX_BO_WRITEBACK,
                             "Printf/abort");