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Re: [Xen-devel] xen questions

To: Scott M Baker <bakers@xxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] xen questions
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>
Date: Tue, 20 Jan 2004 08:48:00 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx, Ian.Pratt@xxxxxxxxxxxx
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> 1) I'm having trouble getting xenctl to work. I'm getting the following
> error message: "Exception in thread "main" java.lang.NoClassDefFoundError:
> org/xml/sax/XMLReader". I've installed jdk-1.4.2 into
> /usr/local/jdk-1.4.2, and ant into /usr/local/ant. 

I'm no java expert, so I'll leave someone else to guess why the
XMLReader library isn't being found.

However, I'd actually advise you to move on to Xen 1.2, which
doesn't use java at all -- all the tools have been rewritten in
python. 1.2 hasn't been officially announced yet, but you can
download it from xen.bkbits.net. I haven't heard reports of any
stability problems with 1.2.
> 2) I'm looking for a way to intercept system calls within the guest O/S
> using the Xen VMM (i.e. when a user-mode application makes a system call,
> I want to intercept it and do something with it before it gets to the
> kernel of the guest O/S). 
> If I understand things correctly, the SYSCALL_VECTOR is filled into
> a trap_info table by the guest O/S, and passed to xen via a
> HYPERVISOR_set_trap_table() call. I would expect that when a trap occurs,
> Xen looks up the trap number, consults the current trap_table, and
> executes the trap. But, I'm having trouble finding that code -- can
> someone point me in the right direction?

Take a look at xen/arxh/i386/traps.c

> 3) What is the "fast_trap" mechanism? I assume it's an optimization of
> Xen's normal trap processing? I'm assuming that I'll have to turn off the
> fast_trap for system calls in order to accomplish #2 above?

Yes, thats correct. fast_trap enables OS syscalls to be delivered
to the OS without bouncing through Xen.

If you can tell us a bit about what you're doing we may be able
to help you -- we've been discussing generic control interfaces
for debugging/migration/checkpointing/fault_injection/interception.


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