ac/virtio: add virtio-only AMDGPU_GEM_CREATE flag

On the host side virglrenderer creates dmabuf on demand when:
* cpu mapping is requested
* setting up scan out
* sharing buffers between guest processes

On-demand dmabuf creation only works if the ctx that created the
BO still exists and knows about this BO. This assumption works ok for
the first 2 cases, but can break with the last one (and it does cause
issues on Android). eg:
* process A allocates BO and exports it as a guest dmabuf
* process A closes its handle to the BO (-> detach_resource)
* process B imports the guest dmabuf -> this triggers the attach_resource
  function in virglrenderer. If the given resource isn't a
  VIRGL_RESOURCE_FD_DMABUF it'll try to get one... But for this to work,
  process A needs to be used -> this fails because this resource was
  detached from it.

The reason we create dmabuf on demand is to avoid hitting the number of
open file descriptor limit. So to cover the 3rd case, we'll use the
VIRTGPU_BLOB_FLAG_USE_SHAREABLE flag, but try to limit to as few possible
buffers as possible.

Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21658>

src/amd/common/sid.h

@@ -403,4 +403,9 @@ enum amd_cmp_class_flags
    P_INFINITY = 1 << 9   // Positive infinity
 };
 
+/* Use the last bit of AMDGPU_GEM_CREATE_* flag as a virtio-only
+ * flag.
+ */
+#define AMDGPU_GEM_CREATE_VIRTIO_SHARED 1u << 31
+
 #endif /* _SID_H */

src/amd/common/virtio/amdgpu_virtio_bo.c

@@ -13,6 +13,7 @@
 #include "util/os_mman.h"
 #include "util/os_time.h"
 #include "util/u_math.h"
+#include "sid.h"
 
 #include <xf86drm.h>
 #include <string.h>
@@ -177,6 +178,11 @@ int amdvgpu_bo_alloc(amdvgpu_device_handle dev,
    if (!(request->flags & AMDGPU_GEM_CREATE_NO_CPU_ACCESS))
       blob_flags |= VIRTGPU_BLOB_FLAG_USE_MAPPABLE;
 
+   if (request->flags & AMDGPU_GEM_CREATE_VIRTIO_SHARED) {
+      blob_flags |= VIRTGPU_BLOB_FLAG_USE_SHAREABLE;
+      req.r.flags &= ~AMDGPU_GEM_CREATE_VIRTIO_SHARED;
+   }
+
    /* blob_id 0 is reserved for the shared memory buffer. */
    assert(req.blob_id > 0);