From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sylvestre Gallon <syl@pmbsd.org>
Subject: Re: How works Xen mmu ?
Date: Thu, 3 Jun 2010 15:16:29 +0200
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	<C82C7886.16838%keir.fraser@eu.citrix.com>
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To: Keir Fraser <keir.fraser@eu.citrix.com>
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On Wed, Jun 2, 2010 at 10:07 PM, Keir Fraser <keir.fraser@eu.citrix.com> wr=
ote:
> On 18/05/2010 09:01, "Sylvestre Gallon" <syl@pmbsd.org> wrote:
>
>> My need is to simply perform an identity mapping at 0xb8000 to allow
>> my operating system to access the vga frammebuffer. So my first idea
>> was to use HYPERVISOR_mmu_update like this :
>>
>> mmu_update_t mmu;
>> uint8_t *buf =3D (uint8_t *) 0xb8000;
>>
>> mmu.ptr =3D 0xb8000 | MMU_MACHPHYS_UPDATE
>> mmu.val =3D 0xb8000
>> HYPERVISOR_mmu_update(&mmu, 1, NULL, DOMID_SELF);
>
> You need to use MMU_NORMAL_PT_UPDATE command, passing a pointer to a pte
> (pagetable entry) in mmu.ptr and the value to write to that pte in mmu.va=
l.
>
> =A0-- Keir
>

Thanks for you response.

If I understand I need to do something like that :

mmu_update_t mmu;

addr =3D get_cr3();                     // getting page directory
addr &=3D 0xfffff000;                   // remove pdir flags
addr =3D ((uint32_t*) addr_[0];     // get address of first page dir
(0xb8000 is in the first pdir)
addr +=3D 0xb8 * 4;                    // 0xb8000 must be the pte 0xb8 into=
 pdir 0

mmu.ptr =3D pte | MMU_NORMAL_PT_UPDATE;
mmu.val =3D 0xb8000 | 0x3;        // 0x3 for pte present and rw flags

HYPERVISOR_mmu_update(&mmu, 1, NULL, DOMID_SELF);

while(42);

But when I try this code I get an unhandled page fault :/

Do I have misunderstood something ?

Thanks a lot for your time,

Cheers,

--=20
Sylvestre Gallon