Re: [Xen-devel] [PATCH] This patch fixes several issues related to vmxassist

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 30 Mar 2006, at 18:02, Li, Xin B wrote:

> > Shouldn't we get the Xen portion of vmxassist help us with that, for
> > example by making the hidden descriptor info (base, limit, etc)
> > available to us? There's already a method for loading that
> > stuff out of
> > Xen, right?
> >
> > Looks to me as though the kludge won't work if you unluckily load a
> > real-mode segment value that happens to also reference a 'big segment'
> > in the currently registered GDT.
> Yes, we may have potential bug here, maybe we should hold this patch 
> and try to find a cleaner way.
> -Xin
In fact, the existing implementation of address() is kludgy. It already 
does tests on the selector value to decide whether it is likely to 
refer to a protected-mode or real-mode segment. Unfortunately the test 
may sometimes yield false positives (selectors that look like they 
could be a valid protected-mode value, but actually it's some arbitrary 
real-mode selector).
I don't know the heritage of that code. I expect someone decided it was 
good enough to be getting on with but maybe now it is time to revisit 
and see if we can implement a watertight version which correctly uses 
hidden segment descriptor state which is readily available when running 
on VMX.
It might be worth pinging Leendert about this and see what he thinks.

 -- Keir


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