Re: kern/45830: reading :dev/mem panics an i386 domU

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

The following reply was made to PR kern/45830; it has been noted by GNATS.

From: Manuel Bouyer <bouyer%antioche.eu.org@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: kern-bug-people%NetBSD.org@localhost, gnats-admin%NetBSD.org@localhost, 
netbsd-bugs%NetBSD.org@localhost,
        manu%NetBSD.org@localhost
Subject: Re: kern/45830: reading :dev/mem panics an i386 domU
Date: Mon, 16 Jan 2012 13:11:53 +0100

 On Sun, Jan 15, 2012 at 07:50:05AM +0000, David Holland wrote:
 >  In what way?
 >  
 >  The same as you can (carefully) use /dev/mem to access objects present
 >  in physical memory that the kernel doesn't know about, you should be
 >  able to use /dev/mem to access objects present in "physical" memory
 >  that the hypervisor puts there but that the kernel nonetheless doesn't
 >  know about. If you do it wrong, the hypervisor punts you off, same as
 
 Because of the way Xen works, if the kernel doesn't know about it
 it doesn't have enough data to ask the hypervisor to map it (this is
 the phys->machine address map). Xen has other ways to map objects the
 kernel doesn't know about (there are some specific hypercalls, and some
 specific files/devices in /kern and /dev for this).
 
 What's happening here is that we're accessing an object that the kernel
 thinks it knows, but the kernel is wrong. I've seen a similar panic
 in other circunstances (not associated with /dev/mem abitrary reads) and
 it may be the same root cause.
 
 -- 
 Manuel Bouyer <bouyer%antioche.eu.org@localhost>
      NetBSD: 26 ans d'experience feront toujours la difference
 --