KVM: MMU: Don't allocate gfns page for direct mmu pages

When sp->role.direct is set, sp->gfns does not contain any essential
information, leaf sptes reachable from this sp are for a continuous
guest physical memory range (a linear range).
So sp->gfns[i] (if it was set) equals to sp->gfn + i. (PT_PAGE_TABLE_LEVEL)
Obviously, it is not essential information, we can calculate it when need.

It means we don't need sp->gfns when sp->role.direct=1,
Thus we can save one page usage for every kvm_mmu_page.

Note:
  Access to sp->gfns must be wrapped by kvm_mmu_page_get_gfn()
  or kvm_mmu_page_set_gfn().
  It is only exposed in FNAME(sync_page).

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/Documentation/kvm/mmu.txt b/Documentation/kvm/mmu.txt
index 0e872ae3091..2201dcba92a 100644
--- a/Documentation/kvm/mmu.txt
+++ b/Documentation/kvm/mmu.txt
@@ -180,7 +180,9 @@ Shadow pages contain the following information:
     guest pages as leaves.
   gfns:
     An array of 512 guest frame numbers, one for each present pte.  Used to
-    perform a reverse map from a pte to a gfn.
+    perform a reverse map from a pte to a gfn. When role.direct is set, any
+    element of this array can be calculated from the gfn field when used, in
+    this case, the array of gfns is not allocated. See role.direct and gfn.
   slot_bitmap:
     A bitmap containing one bit per memory slot.  If the page contains a pte
     mapping a page from memory slot n, then bit n of slot_bitmap will be set