Re: [Xen-devel] Problem setting up a custom GDT

[Mathieu Ropert <mro@xxxxxxxxxx>]

Simon Kagstrom wrote:

> Hello!
>
> I have some problems setting up a custom GDT (I need one where the
> code is readable). Anyway, I try to switch to my own GDT as follows:
>
>   unsigned long frames[16];
>
>   volatile unsigned long va = (unsigned long)GDT;
>   volatile unsigned long pa = to_phys(va);
>   volatile unsigned long ma = phys_to_machine(pa) & ~_PAGE_RW  | _PAGE_PRESENT 
> | _PAGE_ACCESSED | _PAGE_DIRTY;
>
>   /* Make it read-only */
>   HYPERVISOR_update_va_mapping(va, (pte_t){ ma }, 0);
>   frames[0] = ma;
>
>   int ret = HYPERVISOR_set_gdt(frames, 16);
>
>
> (The GDT Is just copied from Linux' right now). When doing this, I
> always get -EINVAL from set_gdt, which I suspect is because
> HYPERVISOR_update_va_mapping for some reason does not succeed in
> setting the page read-only.
>
> I've checked the Linux implementation, and it does basically the same
> thing (pte_wrprotect() on the machine-address before calling
> update_va_mapping). Does anyone see what is wrong here?
>
> // Simon
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel
>  
The machine address you pass to Xen is a page table entry. Xen want a 
machine frame number.
frames[0] = ma >> PAGE_SHIFT;

should fix you problem.

Mathieu

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