powerpc/mm: Move hash specific pte bits to be top bits of RPN

We don't support the full 57 bits of physical address and hence can
overload the top bits of RPN as hash specific pte bits.

Add a BUILD_BUG_ON() to enforce the relationship between H_PAGE_F_SECOND
and H_PAGE_F_GIX.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
[mpe: Move the BUILD_BUG_ON() into hash_utils_64.c and comment it]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index c554768b1fa2..d0ee17029e99 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -981,6 +981,19 @@ void __init hash__early_init_devtree(void)
 
 void __init hash__early_init_mmu(void)
 {
+	/*
+	 * We have code in __hash_page_64K() and elsewhere, which assumes it can
+	 * do the following:
+	 *   new_pte |= (slot << H_PAGE_F_GIX_SHIFT) & (H_PAGE_F_SECOND | H_PAGE_F_GIX);
+	 *
+	 * Where the slot number is between 0-15, and values of 8-15 indicate
+	 * the secondary bucket. For that code to work H_PAGE_F_SECOND and
+	 * H_PAGE_F_GIX must occupy four contiguous bits in the PTE, and
+	 * H_PAGE_F_SECOND must be placed above H_PAGE_F_GIX. Assert that here
+	 * with a BUILD_BUG_ON().
+	 */
+	BUILD_BUG_ON(H_PAGE_F_SECOND != (1ul  << (H_PAGE_F_GIX_SHIFT + 3)));
+
 	htab_init_page_sizes();
 
 	/*