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

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 9/2/26 22:14, Val Packett wrote:
> 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.

Should VIRTIO_GPU_F_CREATE_GUEST_HANDLE require VIRTIO_GPU_F_BLOB_CTX_ID_FIX?
That could simplify drivers.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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