[Xen-users] [XEN 3.3 - PCI passthrough] - interaction latencies with /dev/ttyUSB0

["jer jer" <jeremie.garcia.trash@xxxxxxxxx>]

Hi everybody!
I'm a pretty new XEN user (3.3) and I need your help on a weird situation.
In fact I cannot conclude if it could be a XEN problem or not. So I hope you will be able to guide me.
You will find below what my project is and as you guess ...the problem!


OBJECTIVE :
===========
   - validate XEN solution for potential projects
   - setup a paravirtualized environment 
   - embedd on a domU, software interacting with a USB connected desktop RFID reader


ENVIRONMENT :
=============
   - XEN 3.3
   - paravirtualized environment
   - dom0 : linux 2.6.24-19-xen (ubuntu) installed with no graphical interface 
   - domU : linux 2.6.24-19-xen (ubuntu) installed with gnome and vncserver 
   - PCI passthrough to dedicate all USB controllers to the single domU
   - domU is installed in a file-backed VBDs

BASIC RESULT:
=============
   - dom0 and domU boots well
   - PCI passthrough works
   - system is stable 
   - on domU, lsusb shows the device connected

USB DEVICE :
============
   - RFID desktop reader (STID)
   - serial communication ("/dev/ttyUSB0) 9600 8 n 2
   - very short instructions sent and received asynchronously (5 bytes) ; minimum system stress...

EMBEDDED SOFTWARE :
===================
   - very basic test software that polls the device for an event
   - if event detected, send a sound instruction to the device
   - communication with the device is done via the serial dev file "/dev/ttyUSB0"
   - tested on non virtualized environment : no latency on execution and test succeeds


PROBLEM :
=========
   - running the soft from the console (xm console) without GUI introduces hudge execution latencies
   - if we connect our appliance from a remote PC via vncviewer, gnome is displayed and run the same soft from an Xterm makes the execution much more faster 
     and coud almost be compared to non-virtualized environment perfs.
   - Note that I've tried to connect via SSH without and then with X exporting, and latencies are still noticed
   - it seems that latencies are only seen when interacting with the device
   - Finally, we do not want to embedd any window manager such as gnome/KDE for footprints concerns
   - soft have been written in C and JAVA but results are the same

HELPPPPP:
=========
As you see, it is a very strange problem and I can't find out who's responsible for that :
   - is it a XEN problem?
   - is it a console problem?
   - is it a PCI passthrough problem?
   - does gnome/vnc introduces implicits optimizations?


Furthermore, in order to find some clues: 
   - can you confirm me that XEN 3.3 does not support USB2.0 (not really needed here ...)
   - I often have a system crash connecting USB mass storage devices (even dom0 get frozen) ; is it a known bug? I haven't found anything on forums.
   - based on what I've read (and test) I cannot use newer XEN kernels if I want to keep PCI passthrough working
   - are there some "best practices" to optimize USB passthrough

I've process a lot of diff, compare, readings and I have to tell you I've no more idea to solve this silly problem. Any help is thus welcomed!!

Thanks a lot for your help!

Cordially, 
Jérémie
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