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

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 <>
     NetBSD: 26 ans d'experience feront toujours la difference

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