MIPS: Switch FPU emulator trap to BREAK instruction.

Arguably using the address error handler has always been ugly. But with
processors that handle unaligned loads and stores in hardware the
current mechanism ceases to work so switch it to a BREAK instruction and
allocate break code 514 to the FPU emulator.

Yoichi Yuasa provided a build fix for CONFIG_BUG=n.

Signed-off-by: Ralf Baechle <[e-mail]>
Signed-off-by: Yoichi Yuasa <[e-mail]>

arch/mips/include/asm/break.h

···
 #define _BRK_THREADBP	11	/* For threads, user bp (used by debuggers) */
 #define BRK_BUG		512	/* Used by BUG() */
 #define BRK_KDB		513	/* Used in KDB_ENTER() */
+#define BRK_MEMU	514	/* Used by FPU emulator */
 #define BRK_MULOVF	1023	/* Multiply overflow */
 
 #endif /* __ASM_BREAK_H */

arch/mips/include/asm/fpu_emulator.h

···
 #ifndef _ASM_FPU_EMULATOR_H
 #define _ASM_FPU_EMULATOR_H
 
+#include <asm/break.h>
+#include <asm/inst.h>
+
 struct mips_fpu_emulator_stats {
 	unsigned int emulated;
 	unsigned int loads;
···
 };
 
 extern struct mips_fpu_emulator_stats fpuemustats;
+
+extern int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
+	unsigned long cpc);
+extern int do_dsemulret(struct pt_regs *xcp);
+
+/*
+ * Instruction inserted following the badinst to further tag the sequence
+ */
+#define BD_COOKIE 0x0000bd36	/* tne $0, $0 with baggage */
+
+/*
+ * Break instruction with special math emu break code set
+ */
+#define BREAK_MATH (0x0000000d | (BRK_MEMU << 16))
 
 #endif /* _ASM_FPU_EMULATOR_H */

arch/mips/kernel/traps.c

···
 #include <asm/cpu.h>
 #include <asm/dsp.h>
 #include <asm/fpu.h>
+#include <asm/fpu_emulator.h>
 #include <asm/mipsregs.h>
 #include <asm/mipsmtregs.h>
 #include <asm/module.h>
···
 		break;
 	case BRK_BUG:
 		die_if_kernel("Kernel bug detected", regs);
+		force_sig(SIGTRAP, current);
+		break;
+	case BRK_MEMU:
+		/*
+		 * Address errors may be deliberately induced by the FPU
+		 * emulator to retake control of the CPU after executing the
+		 * instruction in the delay slot of an emulated branch.
+		 *
+		 * Terminate if exception was recognized as a delay slot return
+		 * otherwise handle as normal.
+		 */
+		if (do_dsemulret(regs))
+			return;
+
+		die_if_kernel("Math emu break/trap", regs);
 		force_sig(SIGTRAP, current);
 		break;
 	default:

arch/mips/kernel/unaligned.c

···
 
 asmlinkage void do_ade(struct pt_regs *regs)
 {
-	extern int do_dsemulret(struct pt_regs *);
 	unsigned int __user *pc;
 	mm_segment_t seg;
-
-	/*
-	 * Address errors may be deliberately induced by the FPU emulator to
-	 * retake control of the CPU after executing the instruction in the
-	 * delay slot of an emulated branch.
-	 */
-	/* Terminate if exception was recognized as a delay slot return */
-	if (do_dsemulret(regs))
-		return;
-
-	/* Otherwise handle as normal */
 
 	/*
 	 * Did we catch a fault trying to load an instruction?

arch/mips/math-emu/cp1emu.c

···
 #include <asm/branch.h>
 
 #include "ieee754.h"
-#include "dsemul.h"
 
 /* Strap kernel emulator for full MIPS IV emulation */
 
···
 			/* cop control register rd -> gpr[rt] */
 			u32 value;
 
-			if (ir == CP1UNDEF) {
-				return do_dsemulret(xcp);
-			}
 			if (MIPSInst_RD(ir) == FPCREG_CSR) {
 				value = ctx->fcr31;
 				value = (value & ~0x3) | mips_rm[value & 0x3];

arch/mips/math-emu/dsemul.c

···
 #include <asm/fpu_emulator.h>
 
 #include "ieee754.h"
-#include "dsemul.h"
 
 /* Strap kernel emulator for full MIPS IV emulation */
 
···
 		return SIGBUS;
 
 	err = __put_user(ir, &fr->emul);
-	err |= __put_user((mips_instruction)BADINST, &fr->badinst);
+	err |= __put_user((mips_instruction)BREAK_MATH, &fr->badinst);
 	err |= __put_user((mips_instruction)BD_COOKIE, &fr->cookie);
 	err |= __put_user(cpc, &fr->epc);
 
···
 	/*
 	 * Do some sanity checking on the stackframe:
 	 *
-	 *  - Is the instruction pointed to by the EPC an BADINST?
+	 *  - Is the instruction pointed to by the EPC an BREAK_MATH?
 	 *  - Is the following memory word the BD_COOKIE?
 	 */
 	err = __get_user(insn, &fr->badinst);
 	err |= __get_user(cookie, &fr->cookie);
 
-	if (unlikely(err || (insn != BADINST) || (cookie != BD_COOKIE))) {
+	if (unlikely(err || (insn != BREAK_MATH) || (cookie != BD_COOKIE))) {
 		fpuemustats.errors++;
 		return 0;
 	}

arch/mips/math-emu/dsemul.h

···
-extern int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc);
-extern int do_dsemulret(struct pt_regs *xcp);
-
-/* Instruction which will always cause an address error */
-#define AdELOAD 0x8c000001	/* lw $0,1($0) */
-/* Instruction which will plainly cause a CP1 exception when FPU is disabled */
-#define CP1UNDEF 0x44400001    /* cfc1 $0,$0 undef  */
-
-/* Instruction inserted following the badinst to further tag the sequence */
-#define BD_COOKIE 0x0000bd36 /* tne $0,$0 with baggage */
-
-/* Setup which instruction to use for trampoline */
-#ifdef STANDALONE_EMULATOR
-#define BADINST CP1UNDEF
-#else
-#define BADINST AdELOAD
-#endif