Re: [virtio-comment] [PATCH] virtio-rng: add a control queue

[Cornelia Huck <cohuck@redhat.com>]

On Mon,  3 Aug 2020 15:24:31 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> A control queue is needed to send command to ask the device to
> release the request buffers if it cannot provide data.
> 
> Changes:
> 
>   5.4.2 Virtqueues
>   0 requestq
>   1 controlq
>   controlq only exists if VIRTIO_RNG_F_CTRL_VQ set.
> 
>   5.4.3 Feature bits
>   VIRTIO_RNG_F_CTRL_VQ (0)
>   Device has a control queue
> 
>   5.4.5 Device Initialization
>   Identify and initialize the request virtqueue.
>   If the VIRTIO_RNG_F_CTRL_VQ feature bit is negotiated,
>   identify the control virtqueue.
> 
>   5.4.6.3 Control Virtqueue
>   The driver uses the control virtqueue (if VIRTIO_RNG_F_CTRL_VQ is negotiated)
>   to send commands to control the device.
>   All commands are of the following form:
>   struct virtio_rng_ctrl_hdr {
>          u8 cmd;
>   };
> 
>   5.4.6.3.1 Request Virtqueue Flushing
>   When the device is not able to provide enough entropy to the driver,
>   the driver can be stuck. And for instance, it cannot be removed from the
>   system to switch to another one. The flush command allows the driver to ask
>   the device to release all the buffers sent to the device and to unlock the
>   driver.
>   #define VIRTIO_RNG_CMD_FLUSH 0
> 
> Bug-Link: https://github.com/oasis-tcs/virtio-spec/issues/83
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  content.tex | 38 ++++++++++++++++++++++++++++++++++++--
>  1 file changed, 36 insertions(+), 2 deletions(-)

(...)

> +\paragraph{Request Virtqueue Flushing}\label{sec:Device Types / Entropy Device / Device Operation / Control Virtqueue / Request Virtqueue Flushing}
> +
> +When the device is not able to provide enough entropy to the driver, the driver can be stuck.
> +And for instance, it cannot be removed from the system to switch to another one.

Maybe

"...the driver may be stuck waiting for the device returning buffers,
and cannot be removed..." ?

> +The flush command allows the driver to ask the device to release all the buffers sent to the
> +device and to unlock the driver.

What does "unlock the driver" mean?

You'll probably also want a device normative statement for that, something like

"When the driver sends a flush request, the device MUST release all
outstanding buffers."

> +
> +\begin{lstlisting}
> +#define VIRTIO_RNG_CMD_FLUSH 0
> +\end{lstlisting}

As we (currently) have only that one command, I think we need to
specify that (a) negotiation of the CTRL_VQ feature implies that the
flush command is available, and that (b) any other value of cmd should
(must?) be ignored by the device. (I'd assume that new commands would
require new feature bits.)

> +
>  \section{Traditional Memory Balloon Device}\label{sec:Device Types / Memory Balloon Device}
>  
>  This is the traditional balloon device.  The device number 13 is