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xen/mm: zero PTEs for non-present MFNs in the initial page table 

When constructing the initial page tables, if the MFN for a usable PFN
is missing in the p2m then that frame is initially ballooned out.  In
this case, zero the PTE (as in decrease_reservation() in
drivers/xen/balloon.c).

This is obviously safe instead of having an valid PTE with an MFN of
INVALID_P2M_ENTRY (~0).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
This commit is contained in:
David Vrabel 2012-07-09 11:39:06 +01:00 committed by Konrad Rzeszutek Wilk
parent d095d43e78
commit 66a27dde9a
1 changed files with 8 additions and 1 deletions
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9
arch/x86/xen/mmu.c
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@ -1432,6 +1432,10 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
* Init-time set_pte while constructing initial pagetables, which
* doesn't allow RO page table pages to be remapped RW.
*
* If there is no MFN for this PFN then this page is initially
* ballooned out so clear the PTE (as in decrease_reservation() in
* drivers/xen/balloon.c).
*
* Many of these PTE updates are done on unpinned and writable pages
* and doing a hypercall for these is unnecessary and expensive. At
* this point it is not possible to tell if a page is pinned or not,
@ -1440,7 +1444,10 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
*/
static void __init xen_set_pte_init(pte_t *ptep, pte_t pte)
{
pte = mask_rw_pte(ptep, pte);
if (pte_mfn(pte) != INVALID_P2M_ENTRY)
pte = mask_rw_pte(ptep, pte);
else
pte = __pte_ma(0);
native_set_pte(ptep, pte);
}