[Xen-devel] Re: what's the purpose of linear page table (PML4 entry 258)

[weiming <zephyr.zhao@xxxxxxxxx>]

I find that it give the same pte value as I manually walking the 4 level page table directory. 
So I guest it facilitates the locating of a pte from va. It removes the 4-level looking up process: pgd->pud->pmd->pt->pte
It saves time. Am I right? 

But I still don't know how it works.

On Mon, Mar 31, 2008 at 2:41 AM, weiming <zephyr.zhao@xxxxxxxxx> wrote:

Hi,

When I read the code of do_update_va_mapping, I find that it update the pte entry in the line page table by looking up the line page table via guest_map_l1e() :

 int do_update_va_mapping(unsigned long va, u64 val64,


                          unsigned long flags)

 {

     l1_pgentry_t   val = l1e_from_intpte(val64);


     struct vcpu   *v   = current;


     struct domain *d   = v->domain;


     l1_pgentry_t  *pl1e;

     unsigned long  vmask, bmap_ptr, gl1mfn;

     cpumask_t      pmask;

     int            rc  = 0;


 
     perfc_incr(calls_to_update_va);


 
     if ( unlikely(!__addr_ok(va) && !paging_mode_external(d)) )


         return -EINVAL;

 

     rc = xsm_update_va_mapping(current->domain, val);


     if ( rc )

         return rc;


 
     LOCK_BIGLOCK(d);


 
     pl1e = guest_map_l1e(v, va, &gl1mfn);


...
...

what's the purpose of this table?

Thanks

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