MIPS: kernel: mips-r2-to-r6-emul: Add R2 emulator for MIPS R6

MIPS R6 removed quite a few R2 instructions. However, there
is plenty of <R6 userland code so we add an in-kernel emulator
so we can still be able to execute all R2 userland out there.

The emulator comes with a handy debugfs under /mips/ directory
(r2-emul-stats) to provide some basic statistics of the
instructions that are being emulated.

Below are some statistics from booting a minimal buildroot image:

Instruction     Total   BDslot
------------------------------
movs            236969  0
hilo            56686   0
muls            55279   0
divs            10941   0
dsps            0       0
bops            1       0
traps           0       0
fpus            0       0
loads           214981  17
stores          103364  0
llsc            56898   0
dsemul          150418  0
jr              370158
bltzl           43
bgezl           1594
bltzll          0
bgezll          0
bltzal          39
bgezal          39
beql            14503
bnel            138741
blezl           0
bgtzl           3988

Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>

arch/mips/Kconfig

@@ -2063,6 +2063,19 @@ config MIPS_MT_FPAFF
 	default y
 	depends on MIPS_MT_SMP
 
+config MIPSR2_TO_R6_EMULATOR
+	bool "MIPS R2-to-R6 emulator"
+	depends on CPU_MIPSR6 && !SMP
+	default y
+	help
+	  Choose this option if you want to run non-R6 MIPS userland code.
+	  Even if you say 'Y' here, the emulator will still be disabled by
+	  default. You can enable it using the 'mipsr2emul' kernel option.
+	  The only reason this is a build-time option is to save ~14K from the
+	  final kernel image.
+comment "MIPS R2-to-R6 emulator is only available for UP kernels"
+	depends on SMP && CPU_MIPSR6
+
 config MIPS_VPE_LOADER
 	bool "VPE loader support."
 	depends on SYS_SUPPORTS_MULTITHREADING && MODULES

arch/mips/include/asm/branch.h

@@ -13,9 +13,6 @@
 #include <asm/ptrace.h>
 #include <asm/inst.h>
 
-static int mipsr2_emulation = 0;
-#define NO_R6EMU	(cpu_has_mips_r6 && !mipsr2_emulation)
-
 extern int __isa_exception_epc(struct pt_regs *regs);
 extern int __compute_return_epc(struct pt_regs *regs);
 extern int __compute_return_epc_for_insn(struct pt_regs *regs,

arch/mips/include/asm/mips-r2-to-r6-emul.h

@@ -0,0 +1,96 @@
+/*
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (c) 2014 Imagination Technologies Ltd.
+ * Author: Markos Chandras <markos.chandras@imgtec.com>
+ */
+
+#ifndef __ASM_MIPS_R2_TO_R6_EMUL_H
+#define __ASM_MIPS_R2_TO_R6_EMUL_H
+
+struct mips_r2_emulator_stats {
+	u64 movs;
+	u64 hilo;
+	u64 muls;
+	u64 divs;
+	u64 dsps;
+	u64 bops;
+	u64 traps;
+	u64 fpus;
+	u64 loads;
+	u64 stores;
+	u64 llsc;
+	u64 dsemul;
+};
+
+struct mips_r2br_emulator_stats {
+	u64 jrs;
+	u64 bltzl;
+	u64 bgezl;
+	u64 bltzll;
+	u64 bgezll;
+	u64 bltzall;
+	u64 bgezall;
+	u64 bltzal;
+	u64 bgezal;
+	u64 beql;
+	u64 bnel;
+	u64 blezl;
+	u64 bgtzl;
+};
+
+#ifdef CONFIG_DEBUG_FS
+
+#define MIPS_R2_STATS(M)						\
+do {									\
+	u32 nir;							\
+	int err;							\
+									\
+	preempt_disable();						\
+	__this_cpu_inc(mipsr2emustats.M);				\
+	err = __get_user(nir, (u32 __user *)regs->cp0_epc);		\
+	if (!err) {							\
+		if (nir == BREAK_MATH)					\
+			__this_cpu_inc(mipsr2bdemustats.M);		\
+	}								\
+	preempt_enable();						\
+} while (0)
+
+#define MIPS_R2BR_STATS(M)					\
+do {								\
+	preempt_disable();					\
+	__this_cpu_inc(mipsr2bremustats.M);			\
+	preempt_enable();					\
+} while (0)
+
+#else
+
+#define MIPS_R2_STATS(M)          do { } while (0)
+#define MIPS_R2BR_STATS(M)        do { } while (0)
+
+#endif /* CONFIG_DEBUG_FS */
+
+struct r2_decoder_table {
+	u32     mask;
+	u32     code;
+	int     (*func)(struct pt_regs *regs, u32 inst);
+};
+
+
+extern void do_trap_or_bp(struct pt_regs *regs, unsigned int code,
+			  const char *str);
+
+#ifndef CONFIG_MIPSR2_TO_R6_EMULATOR
+static int mipsr2_emulation;
+static __maybe_unused int mipsr2_decoder(struct pt_regs *regs, u32 inst) { return 0; };
+#else
+/* MIPS R2 Emulator ON/OFF */
+extern int mipsr2_emulation;
+extern int mipsr2_decoder(struct pt_regs *regs, u32 inst);
+#endif /* CONFIG_MIPSR2_TO_R6_EMULATOR */
+
+#define NO_R6EMU	(cpu_has_mips_r6 && !mipsr2_emulation)
+
+#endif /* __ASM_MIPS_R2_TO_R6_EMUL_H */

arch/mips/kernel/Makefile

@@ -90,6 +90,7 @@ obj-$(CONFIG_EARLY_PRINTK)	+= early_printk.o
 obj-$(CONFIG_EARLY_PRINTK_8250)	+= early_printk_8250.o
 obj-$(CONFIG_SPINLOCK_TEST)	+= spinlock_test.o
 obj-$(CONFIG_MIPS_MACHINE)	+= mips_machine.o
+obj-$(CONFIG_MIPSR2_TO_R6_EMULATOR)	+= mips-r2-to-r6-emul.o
 
 CFLAGS_cpu-bugs64.o	= $(shell if $(CC) $(KBUILD_CFLAGS) -Wa,-mdaddi -c -o /dev/null -x c /dev/null >/dev/null 2>&1; then echo "-DHAVE_AS_SET_DADDI"; fi)
 

arch/mips/kernel/branch.c

@@ -16,6 +16,7 @@
 #include <asm/fpu.h>
 #include <asm/fpu_emulator.h>
 #include <asm/inst.h>
+#include <asm/mips-r2-to-r6-emul.h>
 #include <asm/ptrace.h>
 #include <asm/uaccess.h>
 

arch/mips/kernel/traps.c

@@ -46,6 +46,7 @@
 #include <asm/fpu.h>
 #include <asm/fpu_emulator.h>
 #include <asm/idle.h>
+#include <asm/mips-r2-to-r6-emul.h>
 #include <asm/mipsregs.h>
 #include <asm/mipsmtregs.h>
 #include <asm/module.h>
@@ -837,7 +838,7 @@ asmlinkage void do_fpe(struct pt_regs *regs, unsigned long fcr31)
 	exception_exit(prev_state);
 }
 
-static void do_trap_or_bp(struct pt_regs *regs, unsigned int code,
+void do_trap_or_bp(struct pt_regs *regs, unsigned int code,
 	const char *str)
 {
 	siginfo_t info;
@@ -1027,7 +1028,32 @@ asmlinkage void do_ri(struct pt_regs *regs)
 	unsigned int opcode = 0;
 	int status = -1;
 
+	/*
+	 * Avoid any kernel code. Just emulate the R2 instruction
+	 * as quickly as possible.
+	 */
+	if (mipsr2_emulation && cpu_has_mips_r6 &&
+	    likely(user_mode(regs))) {
+		if (likely(get_user(opcode, epc) >= 0)) {
+			status = mipsr2_decoder(regs, opcode);
+			switch (status) {
+			case 0:
+			case SIGEMT:
+				return;
+			case SIGILL:
+				goto no_r2_instr;
+			default:
+				process_fpemu_return(status,
+						     &current->thread.cp0_baduaddr);
+				return;
+			}
+		}
+	}
+
+no_r2_instr:
+
 	prev_state = exception_enter();
+
 	if (notify_die(DIE_RI, "RI Fault", regs, 0, regs_to_trapnr(regs),
 		       SIGILL) == NOTIFY_STOP)
 		goto out;

arch/mips/math-emu/cp1emu.c

@@ -48,6 +48,7 @@
 #include <asm/processor.h>
 #include <asm/fpu_emulator.h>
 #include <asm/fpu.h>
+#include <asm/mips-r2-to-r6-emul.h>
 
 #include "ieee754.h"
 
@@ -68,7 +69,7 @@ static int fpux_emu(struct pt_regs *,
 #define modeindex(v) ((v) & FPU_CSR_RM)
 
 /* convert condition code register number to csr bit */
-static const unsigned int fpucondbit[8] = {
+const unsigned int fpucondbit[8] = {
 	FPU_CSR_COND0,
 	FPU_CSR_COND1,
 	FPU_CSR_COND2,