[Xen-devel] Re: How to intercept interrupts from guest domains

[Mads Bergdal <mbergdal@xxxxxxxxx>]

Keir Fraser wrote:
> On 19/9/06 10:52, "Mads Bergdal" <mbergdal@xxxxxxxxx> wrote:
>
> > > > I am writing my master thesis on virtualization with Xen. I am trying to
> > > >     intercept the hypercalls coming from the guest domains. More
> > > > specific I am trying to determine where in memory a guest domain is
> > > > writing. Does anyone have a hint on where in the code I should try to do
> > > > this?
> > > If you want to know where a guest is writing you need to do more than
> > > intercept hypercalls. You want to intercept memory accesses to, which would
> > > liekly mean you need to run on shadow pagetables and manipulate access
> > > permissions to trap on first access to a page.
> > >
> > >  -- Keir
> > Yes, that sounds reasonable. Do you know where in the code this could be
> > achieved?
> What's the intended purpose? You could perhaps look at the log-dirty shadow
> mode. This is used to track which pages have been modified by the guest -- a
> page which the guest maps writeable is not made writable in the shadow page
> tables until the time of the first write access (when that page is added to
> a 'dirty log' for further processing).
>
> Be warned that modifying the shadow code is rather more difficult than a
> project that would simply involve adding a hook point to every hypercall!
>
>  -- Keir
Thanks for the hints. I really appreciate it.

My main purpose of this is to try to monitor when a guestdomain tries to 
write to a specific address in it's memory. (An address that it should 
not write to) And then get the Hypervisor to notify my userspace 
"surveillance" program about this.
I have spent quite some time now reading the code. I must admit I am a 
bit lost. I am not sure where in the code I should be looking to get 
started.
From what you write above I take it that you think the easiest approach 
 is to hook into the hypercalls? I that case which hypercalls and where?
If not, where should I look to learn he internals of the shadow mode?

Hope I am not wasting your time...

Mads


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