MIPS: Allow R2 CPUs to turn off generation of 'ehb' instructions.

Some CPUs do not need ehb instructions after writing CP0 registers.
By allowing ehb generation to be overridden in
cpu-feature-overrides.h, we can save a few instructions in the TLB
handler hot paths.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/cpu-features.h

@@ -147,6 +147,10 @@
 #define cpu_has_mips_r	(cpu_has_mips32r1 | cpu_has_mips32r2 | \
 			 cpu_has_mips64r1 | cpu_has_mips64r2)
 
+#ifndef cpu_has_mips_r2_exec_hazard
+#define cpu_has_mips_r2_exec_hazard cpu_has_mips_r2
+#endif
+
 /*
  * MIPS32, MIPS64, VR5500, IDT32332, IDT32334 and maybe a few other
  * pre-MIPS32/MIPS53 processors have CLO, CLZ.  For 64-bit kernels

arch/mips/mm/tlbex.c

@@ -259,7 +259,8 @@ static void __cpuinit build_tlb_write_entry(u32 **p, struct uasm_label **l,
 	}
 
 	if (cpu_has_mips_r2) {
-		uasm_i_ehb(p);
+		if (cpu_has_mips_r2_exec_hazard)
+			uasm_i_ehb(p);
 		tlbw(p);
 		return;
 	}