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Re: [Xen-devel] How console data travel in Xen?

To: Anthony Liguori <aliguori@xxxxxxxxxx>
Subject: Re: [Xen-devel] How console data travel in Xen?
From: NAHieu <nahieu@xxxxxxxxx>
Date: Fri, 1 Jul 2005 02:28:22 +0900
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Reply-to: NAHieu <nahieu@xxxxxxxxx>
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On 7/1/05, Anthony Liguori <aliguori@xxxxxxxxxx> wrote:
> NAHieu wrote:
> >Hello,
> >
> >I am trying to investigate in detail how console data go thru the Xen
> >system. Suppose that I am running a domainU, when I type at the
> >domainU console (I connect to the console with xencons or ssh), how
> >data (from keyboard in this case) travel from low level (hardware) to
> >Xen, then to dom0, then to domU?
> >
> >
> There exists a well-known event channel and shared memory page between
> each dom0 and domU which the control tools are responsible for setting
> up (in Xend, that's xcs--in VM-Tools, that's xenctld).  The shared
> memory page is defined by the shared_info_frame which is queriable as
> part of getdomaininfo().  The event channel is chosen by the control
> tools and is almost always 1.
> The event channel and shared memory page make up the control channel.
> The shared memory page is used as a ring queue for messages (see
> /usr/include/xen/io/domain_controller.h) and the event channel is a
> notification message to say when a new message has arrived.
> One of the message types is CMSG_CONSOLE.  It has one message subtype
> CMSG_CONSOLE_DATA.  Console data (input and output) is passed in 60 byte
> chunks using this message type between dom0 and domU.  This is handled
> at drivers/xen/console/console.c in domU (in the kernel tree) and in
> tools/xcs/*.c in dom0 (or xenctld/console.c in VM-Tools).  In VM-Tools,
> the console messages are just put into a queue until a file (usually a
> pty) is associated with the device in which case everything is
> dumped/read from the file.
> The Xend/xcs architecture is a bit more complex.  xcs forwards all
> messages over two sockets to Xend.  Xend then dispatches these messages
> internally.  The messages are then read/written to a socket.  The stuff
> that's unique to the console is mostly handled in
> tools/python/xen/xend/server/console.py

Anthony, thank you for a great detail explaination. These information
is really helpful to me.

But still few questions stand: how do console data go from HW to Xen,
then to dom0? (the above explaination is only about sessions between
dom0 and domU)

Thank you a lot,

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