Re: [Xen-devel] read-only pagetable entries

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

Hi, 

At 12:54 -0500 on 28 Jun (1309265682), Srujan Kotikela wrote:
>    I am trying to see the working of "p2m_change_type". For this I am
> passing the PFN of a variable (computed using virt_to_pfn(&variable)) in a
> kernel module. Then this PFN is being passed to VMM through a hypercall
> (from a HVM guest). In the hypercall handler, I have the following code
> (where PFN == GFN) :
> 
>    struct vcpu *v = current;
> >    struct domain *d = v->domain;
> >
> >     p2m_type_t ot;
> >     mfn_t mfn;
> >
> >
>     mfn = gfn_to_mfn(d, gfn , &ot);        //find the old type of the gfn
> >     printk("MFN  : %lx\n", mfn);
> >     printk("OT   : %d\n", ot);
> >
> >     p2m_change_type(d, gfn, ot, p2m_ram_ro);

You might want to check the return value of p2m_change_type; it does
an atomic compare-exchange so it might not have changed the type if
another CPU changed the p2m entry under your feet.

>    After this I tried to assign a new value to variable (to test if the
> write request is being dropped).  But, the DomU hangs within the kernel
> module (insmod      command is never being completed).
> 
>    I assume this is because I am doing it within a kernel module. Now I am
> going to try passing a user process' variable's address to kernel module and
> pass it further to hypervisor  and repeat the above process. Before that, I
> just want to make sure that I am going in the right direction.

Yes, that looks basically OK to me.  I don't think there's any reason
why being in the kernel would make a difference.  Did you make sure that
no other variables are on the same page as the one you're protecting?

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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