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| -rw-r--r-- | content.tex | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/content.tex b/content.tex index e5fe9c6..374bc02 100644 --- a/content.tex +++ b/content.tex @@ -3223,6 +3223,9 @@ data and outgoing characters are placed in the transmit queue. \item[VIRTIO_CONSOLE_F_MULTIPORT (1)] Device has support for multiple ports; configuration fields nr_ports and max_nr_ports are valid and control virtqueues will be used. + +\item[VIRTIO_CONSOLE_F_EMERG_WRITE (2)] Device has support for emergency write. + Configuration field emerg_wr is valid. \end{description} \subsection{Device configuration layout}\label{sec:Device Types / Console Device / Device configuration layout} @@ -3233,11 +3236,16 @@ data and outgoing characters are placed in the transmit queue. is set, the maximum number of ports supported by the device can be fetched. + If VIRTIO_CONSOLE_F_EMERG_WRITE is set then the driver can use emergency write + to output a single character without initializing virtio queues, or even + acknowledging the feature. + \begin{lstlisting} struct virtio_console_config { le16 cols; le16 rows; le32 max_nr_ports; + le32 emerg_wr; }; \end{lstlisting} @@ -3248,6 +3256,11 @@ native endian of the guest rather than (necessarily) little-endian. \subsection{Device Initialization}\label{sec:Device Types / Console Device / Device Initialization} \begin{enumerate} +\item If the VIRTIO_CONSOLE_F_EMERG_WRITE feature is offered, the + emerg_wr field of the configuration can be written at any time. + Thus it should work for very early boot debugging output as well as + catastophic OS failures (eg. virtio ring corruption). + \item If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver can read the console dimensions from the configuration fields. |