[PATCH v2] arch/powerpc: Dynamically allocate slb_shadow from memblock

[Jeremy Kerr]

Currently, the slb_shadow buffer is our largest symbol:

  [jk@pablo linux]$ nm --size-sort -r -S obj/vmlinux | head -1
  c000000000da0000 0000000000040000 d slb_shadow

- we allocate 128 bytes per cpu; so 256k with NR_CPUS=2048. As we have
constant initialisers, it's allocated in .text, causing a larger vmlinux
image. We may also allocate unecessary slb_shadow buffers (> no. pacas),
since we use the build-time NR_CPUS rather than the run-time nr_cpu_ids.

We could move this to the bss, but then we still have the NR_CPUS vs
nr_cpu_ids potential for overallocation.

This change dynamically allocates the slb_shadow array, during
initialise_pacas(). At a cost of 104 bytes of text, we save 256k of
data:

  [jk@pablo linux]$ size obj/vmlinux{.orig,}
     text     data      bss       dec     hex   filename
  9202795  5244676  1169576  15617047  ee4c17   obj/vmlinux.orig
  9202899  4982532  1169576  15355007  ea4c7f   obj/vmlinux

Tested on pseries.

Signed-off-by: Jeremy Kerr <j...@ozlabs.org>

---
v2: don't drop the endian conversion, use SLB_NUM_BOLTED

---
 arch/powerpc/kernel/paca.c |   32 +++++++++++++++++++++++++-------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/kernel/paca.c b/arch/powerpc/kernel/paca.c
index 9095a6f7..623c356f 100644
--- a/arch/powerpc/kernel/paca.c
+++ b/arch/powerpc/kernel/paca.c
@@ -99,12 +99,28 @@ static inline void free_lppacas(void) { }
  * 3 persistent SLBs are registered here.  The buffer will be zero
  * initially, hence will all be invaild until we actually write them.
  */
-static struct slb_shadow slb_shadow[] __cacheline_aligned = {
-       [0 ... (NR_CPUS-1)] = {
-               .persistent = cpu_to_be32(SLB_NUM_BOLTED),
-               .buffer_length = cpu_to_be32(sizeof(struct slb_shadow)),
-       },
-};
+static struct slb_shadow *slb_shadow;
+
+static void __init allocate_slb_shadows(int nr_cpus, int limit)
+{
+       int size = PAGE_ALIGN(sizeof(struct slb_shadow) * nr_cpus);
+       slb_shadow = __va(memblock_alloc_base(size, PAGE_SIZE, limit));
+       memset(slb_shadow, 0, size);
+}
+
+static struct slb_shadow * __init init_slb_shadow(int cpu)
+{
+       struct slb_shadow *s = &slb_shadow[cpu];
+
+       s->persistent = cpu_to_be32(SLB_NUM_BOLTED);
+       s->buffer_length = cpu_to_be32(sizeof(*s));
+
+       return s;
+}
+
+#else /* CONFIG_PPC_STD_MMU_64 */
+
+static void __init allocate_slb_shadows(int nr_cpus, int limit) { }
 
 #endif /* CONFIG_PPC_STD_MMU_64 */
 
@@ -142,7 +158,7 @@ void __init initialise_paca(struct paca_struct *new_paca, 
int cpu)
        new_paca->__current = &init_task;
        new_paca->data_offset = 0xfeeeeeeeeeeeeeeeULL;
 #ifdef CONFIG_PPC_STD_MMU_64
-       new_paca->slb_shadow_ptr = &slb_shadow[cpu];
+       new_paca->slb_shadow_ptr = init_slb_shadow(cpu);
 #endif /* CONFIG_PPC_STD_MMU_64 */
 }
 
@@ -190,6 +206,8 @@ void __init allocate_pacas(void)
 
        allocate_lppacas(nr_cpu_ids, limit);
 
+       allocate_slb_shadows(nr_cpu_ids, limit);
+
        /* Can't use for_each_*_cpu, as they aren't functional yet */
        for (cpu = 0; cpu < nr_cpu_ids; cpu++)
                initialise_paca(&paca[cpu], cpu);
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