[PATCH v3 3/3] virtio-gpu: advertise VIRTIO_GPU_F_BLOB_ALIGNMENT

[Lucas Amaral <lucaaamaral@gmail.com>]

Define VIRTIO_GPU_F_BLOB_ALIGNMENT (feature bit 5) per the OASIS
virtio specification.  This allows the host to communicate its
memory alignment requirement to the guest via the blob_alignment
field in virtio_gpu_config.

Set blob_alignment to the host page size so the guest kernel can
align blob BAR offsets accordingly, avoiding alignment mismatches
with the VMM's memory mapping API (e.g., hv_vm_map on macOS).

Guest-side kernel support (drm_mm alignment) is pending upstream
Linux merge.

Signed-off-by: Lucas Amaral <lucaaamaral@gmail.com>
---
 hw/display/virtio-gpu.c                     | 8 ++++++++
 include/standard-headers/linux/virtio_gpu.h | 6 ++++++
 2 files changed, 14 insertions(+)

diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
index de7a86a7..44ae5034 100644
--- a/hw/display/virtio-gpu.c
+++ b/hw/display/virtio-gpu.c
@@ -1532,6 +1532,14 @@ void virtio_gpu_device_realize(DeviceState *qdev, Error **errp)
 #endif
     }
 
+    /*
+     * TODO: guest-side F_BLOB_ALIGNMENT support pending upstream Linux merge
+     * (Sergio Lopez's patches, Nov 2025). Until merged, the guest won't
+     * negotiate this feature. The host advertises it per OASIS virtio spec.
+     */
+    g->parent_obj.virtio_config.blob_alignment =
+        cpu_to_le32(qemu_real_host_page_size());
+
     if (!virtio_gpu_base_device_realize(qdev,
                                         virtio_gpu_handle_ctrl_cb,
                                         virtio_gpu_handle_cursor_cb,
diff --git a/include/standard-headers/linux/virtio_gpu.h b/include/standard-headers/linux/virtio_gpu.h
index 00cd3f04..252704e7 100644
--- a/include/standard-headers/linux/virtio_gpu.h
+++ b/include/standard-headers/linux/virtio_gpu.h
@@ -64,6 +64,11 @@
  * context_init and multiple timelines
  */
 #define VIRTIO_GPU_F_CONTEXT_INIT        4
+/*
+ * VIRTIO_GPU_F_BLOB_ALIGNMENT: device advertises blob_alignment
+ * in virtio_gpu_config (OASIS virtio-spec feature bit 5)
+ */
+#define VIRTIO_GPU_F_BLOB_ALIGNMENT      5
 
 enum virtio_gpu_ctrl_type {
 	VIRTIO_GPU_UNDEFINED = 0,
@@ -365,6 +370,7 @@ struct virtio_gpu_config {
 	uint32_t events_clear;
 	uint32_t num_scanouts;
 	uint32_t num_capsets;
+	uint32_t blob_alignment;
 };
 
 /* simple formats for fbcon/X use */
-- 
2.52.0