[POWERPC] 8xx: Work around CPU15 erratum.

The CPU15 erratum on MPC8xx chips can cause incorrect code execution
under certain circumstances, where there is a conditional or indirect
branch in the last word of a page, with a target in the last cache line
of the next page.  This patch implements one of the suggested
workarounds, by forcing a TLB miss whenever execution crosses a page
boundary.  This is done by invalidating the pages before and after the
one being loaded into the TLB in the ITLB miss handler.
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/kernel/head_8xx.S

@@ -298,6 +298,12 @@ InstructionTLBMiss:
 	stw	r10, 0(r0)
 	stw	r11, 4(r0)
 	mfspr	r10, SPRN_SRR0	/* Get effective address of fault */
+#ifdef CONFIG_8xx_CPU15
+	addi	r11, r10, 0x1000
+	tlbie	r11
+	addi	r11, r10, -0x1000
+	tlbie	r11
+#endif
 	DO_8xx_CPU6(0x3780, r3)
 	mtspr	SPRN_MD_EPN, r10	/* Have to use MD_EPN for walk, MI_EPN can't */
 	mfspr	r10, SPRN_M_TWB	/* Get level 1 table entry address */

arch/powerpc/platforms/8xx/Kconfig

@@ -100,6 +100,22 @@ config 8xx_CPU6
 
 	  If in doubt, say N here.
 
+config 8xx_CPU15
+	bool "CPU15 Silicon Errata"
+	default y
+	help
+	  This enables a workaround for erratum CPU15 on MPC8xx chips.
+	  This bug can cause incorrect code execution under certain
+	  circumstances.  This workaround adds some overhead (a TLB miss
+	  every time execution crosses a page boundary), and you may wish
+	  to disable it if you have worked around the bug in the compiler
+	  (by not placing conditional branches or branches to LR or CTR
+	  in the last word of a page, with a target of the last cache
+	  line in the next page), or if you have used some other
+	  workaround.
+
+	  If in doubt, say Y here.
+
 choice
 	prompt "Microcode patch selection"
 	default NO_UCODE_PATCH