Re: [Xen-devel] Disable paging in xen kernel

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.06.12 at 08:04, Zeinab Alebouyeh <z.alebouyeh@xxxxxxxxx> wrote:
> I want to write a program in xen that runs in ring 0 and in unpaged
> protected mode.
> For this,  I add a hypercall and in my hypercall function write my code. I
> test my hypercall, it works properly.
> I see the value of CR0 register and I found that the PE and PG bits are 1.
> It means that I am in protected mode with paging enabled.
> In order to disable paging I want to set bit 31 of cr0 to 0, I write the
> bellowing code in my hypercall function:
> 
> asm volatile(
> "movl %cr0,%eax\n\t"
> "and 0x7fffffff,%eax\n\t"
> "movl %eax,%cr0"
> );
> But when I invoke my hypercall the system restart!!!

Of course, as this causes a triple fault without a lot of other
things taken care of.

> Can anyone tell me where is my fault and how should I disable paging in xen
> kernel?

Your fault is of conceptual nature - you just can't do what you're
intending to do in a pre-existing OS or OS-like environment. You
should instead consider to write your own OS-like environment
(started via some boot loader), where either you never enable
paging, or have a way to cleanly turn it off when you need to.

Jan


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