Re: [Xen-devel] xm dmesg and performance on serial port

[kuas <ku4s@xxxxxxxxxxxxxxxxxxxxx>]

> > I might only have configuration problem, but I couldn't find any docs
> > that could relate to the problem. The command: 'xm dmesg' seems to stop
> > buffering any output after a while. I have to enable serial port to get
> > the output bigger than the buffer. I don't know how big the buffer or if
> > it's circular? Is there anyway to fix this behavior?
> >    
> It's not currently circular.  There used to be a -c flag to xm dmesg (or it's 
> equivalent) that cleared the buffer - does that work?  There's code somewhere 
> in Xen which could support a circular buffer with not much effort.
>  
It is not circular or the circular mechanism might have been broken. The 
buffer just stopped after a certain limit. The code in "console.c" does 
not seems to be circular either. And the command with "xm dmesg -c" 
doesn't seem to exist either.
> > Just by reading the XEN papers, XEN has a way to register asynchronous
> > events to the domains. Would it be better in terms of performance to
> > send the event messages to a particular domain (e.g: Domain 0) using the
> > asynchronous mechanism and have the domain kernel to handle the messages
> > as XEN events.
> >    
> We don't use xm dmesg that much for debugging, generally just tracing through 
> the code and using an attached serial line.
> You might want to look at the tracebuffer and the xentrace tools.  The 
> tracebuffer allows you to log events within Xen itself with low overhead into 
> a circular buffer.  The xentrace command in dom0 will read this data out and 
> decode it according to a format file.  There's man pages for the xentrace* 
> tools and the docs/interface.tex manual (although mostly out of date) 
> explains how to put trace points into the Xen code.  
xentrace seems to be used for debugging purpose. I am actually doing a 
security project using Xen. On a particular event that happens in the 
one of the domain, detected by a module in XEN. I'd like to report it or 
log it. I am trying to find the best way to do this. Let me know if I'm 
wrong. Isn't the "event_channel" is the communication channel from XEN 
to a domain. If I want to get XEN buffer messages (I guess it has to be 
in shared pages with the domain) to privilege domain 0, and flag the 
domain to check if there's something in the buffer. And get a handler in 
domain 0 to consume the buffer. I think this would have much less 
performance effect. Is there any interface existed that I can use for 
this? Or I'll just have to built it on top of event_channel.
Thanks,
Kuas.



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