[PATCH v2 2/2] virtio-gpu: add flag to negotiate always passing ctx_id to RESOURCE_CREATE_BLOB

[Val Packett <val@invisiblethingslab.com>]

virtio-gpu device backends typically use ctx_id to route commands to
different modules that implement distinct context types, e.g.
virglrenderer vs. cross-domain. Each module may require the blob
resources intented to use with it to be allocated through it as well.

Currently, this happens for host-only and default blobs which are
allocated from a context-specific local blob_id (e.g. an image
requirements blob in cross-domain). For guest-only resources however,
existing software (both the Linux kernel and the rutabaga-gfx library
used by device backends to implement virtio-gpu) allocates those with
ctx_id == 0, which might be a different "default" context type from the
one the resource is intended to be used with, e.g. when both
virglrenderer and cross-domain are present, the socket buffers used by
cross-domain were processed by virglrenderer which happens to work
but does not actually make sense.

Additionally we would like to support PRIME imported resources (udmabufs
or dGPU passthrough dma-bufs) being passed over cross-domain with the
CREATE_GUEST_HANDLE feature, which due to API constraints must also
perform the creation of a guest-only blob resource with ctx_id but
without blob_id.

Unfortunately changing this without breaking backwards compatibility
requires a flag to negotiate the use of the "fixed" convention. Otherwise
new guest kernel + old rutabaga would result in the command failing due
to old rutabaga not expecting guest resources to be allocated w/ ctx_id.

Signed-off-by: Val Packett <val@invisiblethingslab.com>
---
 device-types/gpu/description.tex | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/device-types/gpu/description.tex b/device-types/gpu/description.tex
index 64e8c4ddecc5..c0ff3f34ec1c 100644
--- a/device-types/gpu/description.tex
+++ b/device-types/gpu/description.tex
@@ -42,6 +42,9 @@ \subsection{Feature bits}\label{sec:Device Types / GPU Device / Feature bits}
 \item[VIRTIO_GPU_F_CREATE_GUEST_HANDLE (6)] guest-only blob resources
   may be created with the VIRTIO_GPU_BLOB_FLAG_CREATE_GUEST_HANDLE flag.
   Requires VIRTIO_GPU_F_RESOURCE_BLOB.
+\item[VIRTIO_GPU_F_BLOB_CTX_ID_FIX (7)] it is always safe to pass the
+  current context ID to VIRTIO_GPU_CMD_RESOURCE_CREATE_BLOB,
+  including for guest-only blobs.
 \end{description}
 
 \subsection{Device configuration layout}\label{sec:Device Types / GPU Device / Device configuration layout}
@@ -673,6 +676,12 @@ \subsubsection{Device Operation: controlq}\label{sec:Device Types / GPU Device /
 identified by the \field{blob_id}. The actual allocation is done via
 VIRTIO_GPU_CMD_SUBMIT_3D.
 
+If VIRTIO_GPU_F_BLOB_CTX_ID_FIX has been negotiated, the \field{ctx_id} of
+the \field{hdr} MUST always be filled in with the ID of the default rendering
+context associated with the current handle, if one exists. Otherwise, it MUST
+only be filled when the \field{blob_id} has been filled to create a resource
+from a rendering context local object.
+
 The driver MUST inform the device if the blob resource is used for
 memory access, sharing between driver instances and/or sharing with
 other devices. This is done via the \field{blob_flags} field.
-- 
2.55.0