Re: How console data travel in Xen?

[Anthony Liguori <aliguori@us.ibm.com>]

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

Hope this helps.

Regards,

Anthony Liguori

>I am not afraid of reading and hacking the source to understand this,
>but I dont know where to start in the jungle of code. (Which files
>should I pay attention to?)
>
>Looks like xen/drivers/char/console.c is not related to this problem?
>
>All the helps are highly appreciated ;-)
>
>Thank you a lot,
>Hieu
>
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>  
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