powerpc/8xx: Map linear kernel RAM with 8M pages

On a live running system (VoIP gateway for Air Trafic Control), over
a 10 minutes period (with 277s idle), we get 87 millions DTLB misses
and approximatly 35 secondes are spent in DTLB handler.
This represents 5.8% of the overall time and even 10.8% of the
non-idle time.
Among those 87 millions DTLB misses, 15% are on user addresses and
85% are on kernel addresses. And within the kernel addresses, 93%
are on addresses from the linear address space and only 7% are on
addresses from the virtual address space.

MPC8xx has no BATs but it has 8Mb page size. This patch implements
mapping of kernel RAM using 8Mb pages, on the same model as what is
done on the 40x.

In 4k pages mode, each PGD entry maps a 4Mb area: we map every two
entries to the same 8Mb physical page. In each second entry, we add
4Mb to the page physical address to ease life of the FixupDAR
routine. This is just ignored by HW.

In 16k pages mode, each PGD entry maps a 64Mb area: each PGD entry
will point to the first page of the area. The DTLB handler adds
the 3 bits from EPN to map the correct page.

With this patch applied, we now get only 13 millions TLB misses
during the 10 minutes period. The idle time has increased to 313s
and the overall time spent in DTLB miss handler is 6.3s, which
represents 1% of the overall time and 2.2% of non-idle time.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>

1 files changed, 3 insertions, 12 deletions

diff --git a/arch/powerpc/mm/mmu_decl.h b/arch/powerpc/mm/mmu_decl.h
index 898d63365cdd..718076ff0b8a 100644
--- a/arch/powerpc/mm/mmu_decl.h
+++ b/arch/powerpc/mm/mmu_decl.h
@@ -133,22 +133,17 @@ extern void wii_memory_fixups(void);
 /* ...and now those things that may be slightly different between processor
  * architectures.  -- Dan
  */
-#if defined(CONFIG_8xx)
-#define MMU_init_hw()		do { } while(0)
-#define mmu_mapin_ram(top)	(0UL)
-
-#elif defined(CONFIG_4xx)
+#ifdef CONFIG_PPC32
 extern void MMU_init_hw(void);
 extern unsigned long mmu_mapin_ram(unsigned long top);
+#endif
 
-#elif defined(CONFIG_PPC_FSL_BOOK3E)
+#ifdef CONFIG_PPC_FSL_BOOK3E
 extern unsigned long map_mem_in_cams(unsigned long ram, int max_cam_idx,
 				     bool dryrun);
 extern unsigned long calc_cam_sz(unsigned long ram, unsigned long virt,
 				 phys_addr_t phys);
 #ifdef CONFIG_PPC32
-extern void MMU_init_hw(void);
-extern unsigned long mmu_mapin_ram(unsigned long top);
 extern void adjust_total_lowmem(void);
 extern int switch_to_as1(void);
 extern void restore_to_as0(int esel, int offset, void *dt_ptr, int bootcpu);
@@ -163,8 +158,4 @@ struct tlbcam {
 	u32	MAS3;
 	u32	MAS7;
 };
-#elif defined(CONFIG_PPC32)
-/* anything 32-bit except 4xx or 8xx */
-extern void MMU_init_hw(void);
-extern unsigned long mmu_mapin_ram(unsigned long top);
 #endif