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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Oct 16, 2026 at 04:47:36PM +0800, Linlin Zhang wrote:
> 
> 
> On 10/14/2026 5:43 PM, Michael S. Tsirkin wrote:
> > 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,
> >> +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.
> >> +
> >>  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
> > 
> > let the driver control buffering.
> > you want the status as 1st byte of input. Then driver can separate
> > it in a buffer of its own.
> 
> Thanks, that makes sense.
> 
> I'll re-word it as the following:
> 
>   - Control virtqueue requests consist of a type field followed by optional
>     command-specific device-readable data, and a status byte followed by
>     optional command-specific device-writable data.

ok

>     The status byte MUST be the first byte of the device-writable portion of the request.

not that informative and stuff like this belongs in a conformance
section anyway. 

> > 
> >> -- 
> >> 2.34.1
> >>
> >