Re: [PATCH v3 1/2] virtio-blk: Add the control virtqueue

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sun, Sep 13, 2026 at 09:16:14AM -0700, Linlin Zhang wrote:
> From: linlzhan <linlin.zhang@oss.qualcomm.com>
> 
> Add VIRTIO_BLK_F_CTRL_VQ to advertise control virtqueue support.
> 
> Define the control virtqueue location, driver queue discovery,
> and common control request buffer layout. This provides a
> standard framework for block-device control commands.
> 
> Signed-off-by: linlzhan <linlin.zhang@oss.qualcomm.com>
> ---
>  device-types/blk/description.tex | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/device-types/blk/description.tex b/device-types/blk/description.tex
> index 3b3a4e7..9bfdc4a 100644
> --- a/device-types/blk/description.tex
> +++ b/device-types/blk/description.tex
> @@ -13,11 +13,16 @@ \subsection{Virtqueues}\label{sec:Device Types / Block Device / Virtqueues}
>  \item[0] requestq1
>  \item[\ldots]
>  \item[N-1] requestqN
> +\item[N] controlq, if VIRTIO_BLK_F_CTRL_VQ is negotiated
>  \end{description}
>  
>   N=1 if VIRTIO_BLK_F_MQ is not negotiated, otherwise N is set by
>   \field{num_queues}.
>  
> +If VIRTIO_BLK_F_CTRL_VQ is negotiated, the control virtqueue is appended
> +after the request virtqueues. The control virtqueue is reserved for control
> +requests defined by this specification.
> +
>  \subsection{Feature bits}\label{sec:Device Types / Block Device / Feature bits}
>  
>  \begin{description}
> @@ -73,6 +78,8 @@ \subsection{Feature bits}\label{sec:Device Types / Block Device / Feature bits}
>      VIRTIO_BLK_REQ_FLAG_OUT_FUA flag in the \field{flags} bitfield of the
>      \field{virtio_blk_req} structure for VIRTIO_BLK_T_OUT requests.
>  
> +\item[VIRTIO_BLK_F_CTRL_VQ (22)] Device supports a control virtqueue.
> +
>  \end{description}
>  
>  \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Block Device / Feature bits / Legacy Interface: Feature bits}
> @@ -274,6 +281,9 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
>  \item If the VIRTIO_BLK_F_MQ feature is negotiated, \field{num_queues} field
>      can be read to determine the number of queues.
>  
> +\item If the VIRTIO_BLK_F_CTRL_VQ feature is negotiated, the driver MUST
> +    identify the control virtqueue as queue N, after all request virtqueues.
> +
>  \item If the VIRTIO_BLK_F_SECURE_ERASE feature is negotiated,
>      \field{max_secure_erase_sectors} and \field{max_secure_erase_seg} can be read
>      to determine the maximum secure erase sectors and maximum number of
> @@ -327,6 +337,10 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
>  The driver MUST NOT negotiate VIRTIO_BLK_F_REQ_FLAGS_OUT_FUA without
>  VIRTIO_BLK_F_REQ_FLAGS.
>  
> +If the VIRTIO_BLK_F_CTRL_VQ feature is negotiated, the driver MUST NOT submit
> +any request to the control virtqueue unless the request is defined by this
> +specification.
> +
>  \devicenormative{\subsubsection}{Device Initialization}{Device Types / Block Device / Device Initialization}
>  
>  Devices SHOULD always offer VIRTIO_BLK_F_FLUSH, and MUST offer it
> @@ -454,6 +468,11 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
>  };
>  \end{lstlisting}
>  
> +Control virtqueue requests consist of a type field in an output buffer,

Please move this paragraph to a separate subsection dedicate the control
virtqueue (like virtio-net's 5.1.9.5 Control Virtqueue in VIRTIO 1.4,
https://docs.oasis-open.org/virtio/virtio/v1.4/virtio-v1.4.html#x1-2850009:~:text=5%2E1%2E9%2E5%20Control%20Virtqueue).

I recommend defining a separate struct virtio_blk_ctrl similar to
virtio-net's struct virtio_net_ctrl to illustrate the layout more
clearly than a paragraph of text could.

> +followed by an optional command-specific output buffer, an optional
> +command-specific input buffer, and a status byte in an input buffer. The
> +type field and status byte MUST each be in their own buffer.

Message framing is up to the driver and the device spec cannot dictate
specific framing:
https://docs.oasis-open.org/virtio/virtio/v1.4/virtio-v1.4.html#x1-390004

> +
>  The type of the request is either a read (VIRTIO_BLK_T_IN), a write
>  (VIRTIO_BLK_T_OUT), a discard (VIRTIO_BLK_T_DISCARD), a write zeroes
>  (VIRTIO_BLK_T_WRITE_ZEROES), a flush (VIRTIO_BLK_T_FLUSH), a get device ID
> -- 
> 2.34.1
> 

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