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authorrusty <rusty@0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652>2014-01-29 02:26:01 +0000
committerrusty <rusty@0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652>2014-01-29 02:26:01 +0000
commit6ba5d27b7c891a1fe82de208c2272a536001d10e (patch)
tree23858599333a1f4452da39b1199d4956a6c322da
parent077674922e4f1427b6e31dadc8b89c730adc6b35 (diff)
Feedback #3: Feedback from Pranavkumar Sawargaonkar (VIRTIO_CONSOLE_F_EMERG_WRITE)
Document: virtio-v1.0-csprd01 Number: 3 Date: Tue, 21 Jan 2014 15:09:54 +0530 Link to Mail: https://lists.oasis-open.org/archives/virtio-comment/201401/msg00037.html Commenter name: Pranavkumar Sawargaonkar <pranavkumar@linaro.org> Approved at meeting 2014-01-28: https://lists.oasis-open.org/archives/virtio/201401/msg00054.html Signed-off-by: Rusty Russell <rusty@au1.ibm.com> git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio@197 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
-rw-r--r--content.tex13
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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.