Re: How works Xen mmu ?

[Sylvestre Gallon <syl@pmbsd.org>]

On Wed, Jun 2, 2010 at 10:07 PM, Keir Fraser <keir.fraser@eu.citrix.com> wrote:
> 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 = (uint8_t *) 0xb8000;
>>
>> mmu.ptr = 0xb8000 | MMU_MACHPHYS_UPDATE
>> mmu.val = 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.val.
>
>  -- Keir
>

Thanks for you response.

If I understand I need to do something like that :

mmu_update_t mmu;

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

mmu.ptr = pte | MMU_NORMAL_PT_UPDATE;
mmu.val = 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,

-- 
Sylvestre Gallon