I think users will enable tdp when their hardwares support ept or npt.
This patch can reduce about 50% kvm mmu memory usage for they.
This simple patch use the fact that:
When sp->role.direct is set, sp->gfns does not contain any essential
information, leaf sptes reachable from this sp are for a continuate
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).