arm: add SDIV/UDIV emulation for ARMv7 processors

The Virtualization Extensions introduce the requirement for an ARMv7-A
implementation to include SDIV and UDIV.  Any implementation of the
Virtualization Extensions must include the SDIV and UDIV instructions
in the Thumb and ARM instruction sets.

In an ARMv7-A implementation that does not include the Virtualization
Extensions, it is IMPLEMENTATION DEFINED whether:
 *  SDIV and UDIV are not implemented
 *  SDIV and UDIV are implemented only in the Thumb instruction set
 *  SDIV and UDIV are implemented in the Thumb and ARM instruction sets.

This patch adds a handler to trap and emulate unimplemented SDIV and
UDIV instructions in ARM and Thumb modes. Also some basic statistic is
exported via /proc/cpu/idiv_emulation

Change-Id: I8e721ecac62a05fab42ed7db7951b4c837a59bc7
Signed-off-by: Vladimir Murzin <murzin.v@gmail.com>

3 files changed, 222 insertions, 0 deletions

diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
index 6af1147..2d8dfa1 100644
--- a/arch/arm/kernel/Makefile
+++ b/arch/arm/kernel/Makefile
@@ -49,6 +49,7 @@ obj-$(CONFIG_OF)		+= devtree.o
 obj-$(CONFIG_CRASH_DUMP)	+= crash_dump.o
 obj-$(CONFIG_SWP_EMULATE)	+= swp_emulate.o
 CFLAGS_swp_emulate.o		:= -Wa,-march=armv7-a
+obj-$(CONFIG_IDIV_EMULATE)	+= idiv_emulate.o
 obj-$(CONFIG_HAVE_HW_BREAKPOINT)	+= hw_breakpoint.o
 
 obj-$(CONFIG_CRUNCH)		+= crunch.o crunch-bits.o
diff --git a/arch/arm/kernel/idiv_emulate.c b/arch/arm/kernel/idiv_emulate.c
new file mode 100644
index 0000000..e5818db
--- /dev/null
+++ b/arch/arm/kernel/idiv_emulate.c
@@ -0,0 +1,195 @@
+/*
+ * linux/arch/arm/kernel/idiv_emulate.c
+ *
+ * This code is based on swp_emulate.c
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * Implements emulation of the SDIV/UDIV instructions. They are defined for
+ * ARMv7-R and ARMv7-M profiles (in Thumb state only) and are UNDEFINED in the
+ * ARMv7-A profile. SDIV/UDIV are present by default on Cortex-A15.
+ *
+ * This emulation allow using integer divide instructions in case hardware is
+ * not presented
+ *
+ *  Syntax of SDIV/UDIV instructions: SDIV/UDIV<c> <Rd>,<Rn>,<Rm>
+ *  Where: Rd  = the destination register.
+ *	   Rn  = the register that contains the dividend.
+ *	   Rm  = the register that contains the divisor.
+ */
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/proc_fs.h>
+#include <linux/sched.h>
+#include <linux/syscalls.h>
+#include <linux/perf_event.h>
+
+#include <asm/opcodes.h>
+#include <asm/traps.h>
+#include <asm/uaccess.h>
+
+/*
+ * Macros/defines for extracting register numbers from instruction.
+ */
+#define EXTRACT_REG_NUM(instruction, offset) \
+	(((instruction) & (0xf << (offset))) >> (offset))
+
+/*
+ * Offsets for ARM mode
+ */
+#define RD_OFFSET  16
+#define RN_OFFSET  0
+#define RM_OFFSET  8
+
+/*
+ * Offsets for Thumb mode
+ */
+#define RD_T_OFFSET  8
+#define RN_T_OFFSET  16
+#define RM_T_OFFSET  0
+
+#define TYPE_OF_DIV (1 << 21)
+
+static unsigned long sdivcounter;
+static unsigned long udivcounter;
+static pid_t         previous_pid;
+
+#ifdef CONFIG_PROC_FS
+static int proc_read_status(char *page, char **start, off_t off, int count,
+			    int *eof, void *data)
+{
+	char *p = page;
+	int len;
+
+	p += sprintf(p, "Emulated UDIV:\t\t%lu\n", udivcounter);
+	p += sprintf(p, "Emulated SDIV:\t\t%lu\n", sdivcounter);
+	if (previous_pid != 0)
+		p += sprintf(p, "Last process:\t\t%d\n", previous_pid);
+
+	len = (p - page) - off;
+	if (len < 0)
+		len = 0;
+
+	*eof = (len <= count) ? 1 : 0;
+	*start = page + off;
+
+	return len;
+}
+#endif
+
+static u32 emulate_udiv(u32 n, u32 base)
+{
+	udivcounter++;
+
+	return n/base;
+}
+
+static s32 emulate_sdiv(s32 n, s32  base)
+{
+	sdivcounter++;
+
+	return n/base;
+}
+
+static int idiv_handler(struct pt_regs *regs, unsigned int instr)
+{
+	long dividend, divisor, dest, res;
+
+	perf_sw_event(PERF_COUNT_SW_EMULATION_FAULTS, 1, 0, regs, regs->ARM_pc);
+
+	res = arm_check_condition(instr, regs->ARM_cpsr);
+
+	switch (res) {
+	case ARM_OPCODE_CONDTEST_PASS:
+		break;
+	case ARM_OPCODE_CONDTEST_FAIL:
+		/* Condition failed - return to next instruction */
+		regs->ARM_pc += 4;
+		return 0;
+	case ARM_OPCODE_CONDTEST_UNCOND:
+		if (!thumb_mode(regs))
+			return -EFAULT;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	if (current->pid != previous_pid) {
+		pr_debug("\"%s\" (%ld) uses idiv instruction\n",
+			 current->comm, (unsigned long)current->pid);
+		previous_pid = current->pid;
+	}
+
+	if (!thumb_mode(regs)) {
+		dividend = regs->uregs[EXTRACT_REG_NUM(instr, RN_OFFSET)];
+		divisor = regs->uregs[EXTRACT_REG_NUM(instr, RM_OFFSET)];
+		dest = EXTRACT_REG_NUM(instr, RD_OFFSET);
+	} else {
+		dividend = regs->uregs[EXTRACT_REG_NUM(instr, RN_T_OFFSET)];
+		divisor = regs->uregs[EXTRACT_REG_NUM(instr, RM_T_OFFSET)];
+		dest = EXTRACT_REG_NUM(instr, RD_T_OFFSET);
+	}
+
+/*
+ * In an ARMv7-A profile implementation that supports the SDIV and UDIV
+ * instructions, divide-by-zero always returns a zero result.
+ * In fact, integer division emulation provided by gcc lib has already handle
+ * division by zero case sending the signal to the caused process. Emulate this
+ * behavior here as well.
+ */
+	if (!divisor) {
+		siginfo_t info;
+
+		info.si_code = FPE_INTDIV;
+		info.si_signo = SIGFPE;
+		info.si_errno = 0;
+
+		arm_notify_die("Division by zero", regs, &info, 0, 0);
+
+		goto out;
+	}
+
+	if (instr & TYPE_OF_DIV)
+		res = emulate_udiv((u32)dividend, (u32)divisor);
+	else
+		res = emulate_sdiv((s32)dividend, (s32)divisor);
+
+	regs->ARM_pc += 4;
+	regs->uregs[dest] = res;
+
+out:
+	return 0;
+}
+
+static struct undef_hook idiv_hook = {
+	.instr_mask = 0x0310f010,
+	.instr_val  = 0x0310f010,
+	.cpsr_mask  = MODE_MASK,
+	.cpsr_val   = USR_MODE,
+	.fn	    = idiv_handler
+};
+
+static int __init idiv_emulation_init(void)
+{
+#ifdef CONFIG_PROC_FS
+	struct proc_dir_entry *res;
+
+	res = create_proc_entry("cpu/idiv_emulation", S_IRUGO, NULL);
+
+	if (!res)
+		return -ENOMEM;
+
+	res->read_proc = proc_read_status;
+#endif /* CONFIG_PROC_FS */
+
+	pr_notice("Registering SDIV/UDIV emulation handler\n");
+
+	register_undef_hook(&idiv_hook);
+
+	return 0;
+}
+
+late_initcall(idiv_emulation_init);
diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index cbab5c5..ebaee8e 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -687,6 +687,32 @@ config SWP_EMULATE
 
 	  If unsure, say Y.
 
+config IDIV_EMULATE
+	bool "Emulate SDIV/UDIV instructions"
+	depends on !CPU_USE_DOMAINS && CPU_V7
+	select HAVE_PROC_CPU if PROC_FS
+	default y if SMP
+	help
+
+	  The Virtualization Extensions introduce the requirement for an
+	  ARMv7-A implementation to include SDIV and UDIV.  Any
+	  implementation of the Virtualization Extensions must include
+	  the SDIV and UDIV instructions in the Thumb and ARM
+	  instruction sets.
+
+	  In an ARMv7-A implementation that does not include the
+	  Virtualization Extensions, it is IMPLEMENTATION DEFINED whether:
+	  *  SDIV and UDIV are not implemented
+	  *  SDIV and UDIV are implemented only in the Thumb instruction set
+	  *  SDIV and UDIV are implemented in the Thumb and ARM
+	  instruction sets.
+
+	  This option allows to handle exeptions due to unimplemented
+	  SDIV and UDIV instructions in ARM and Thumb modes.
+
+	  If unsure, say Y.
+
+
 config CPU_BIG_ENDIAN
 	bool "Build big-endian kernel"
 	depends on ARCH_SUPPORTS_BIG_ENDIAN