[PATCH 2/4] powerpc/64s: Add slb_full_bitmap rather than hard-coding U32_MAX

[Michael Ellerman]

The recent rewrite of the SLB code into C included the assumption that
all CPUs we run on have at least 32 SLB entries. This is currently
true but a bit fragile as the SLB size is actually defined by the
device tree and so could theoretically change at any time.

The assumption is encoded in the fact that we use U32_MAX as the value
for a full SLB bitmap. Instead, calculate what the full bitmap would
be based on the SLB size we're given and store it. This still requires
the SLB size to be a power of 2.

Fixes: 126b11b294d1 ("powerpc/64s/hash: Add SLB allocation status bitmaps")
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
---
 arch/powerpc/mm/slb.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
index bc3914d54e26..61450a9cf30d 100644
--- a/arch/powerpc/mm/slb.c
+++ b/arch/powerpc/mm/slb.c
@@ -506,9 +506,16 @@ void switch_slb(struct task_struct *tsk, struct mm_struct *mm)
 	asm volatile("isync" : : : "memory");
 }
 
+static u32 slb_full_bitmap;
+
 void slb_set_size(u16 size)
 {
 	mmu_slb_size = size;
+
+	if (size >= 32)
+		slb_full_bitmap = U32_MAX;
+	else
+		slb_full_bitmap = (1ul << size) - 1;
 }
 
 void slb_initialize(void)
@@ -611,7 +618,7 @@ static enum slb_index alloc_slb_index(bool kernel)
 	 * POWER7/8/9 have 32 SLB entries, this could be expanded if a
 	 * future CPU has more.
 	 */
-	if (local_paca->slb_used_bitmap != U32_MAX) {
+	if (local_paca->slb_used_bitmap != slb_full_bitmap) {
 		index = ffz(local_paca->slb_used_bitmap);
 		local_paca->slb_used_bitmap |= 1U << index;
 		if (kernel)
-- 
2.20.1