[ARM PATCH] 2327/1: Thumb ld/st alignment fault fixups

Patch from George G. Davis

Adds support for Thumb ld/st alignment fault fixups via conversion of
Thumb ld/st instruction forms into equivalent ARM instructions and
reusing ARM alignment fault handler for Thumb instructions. This
implementation was inspired by and initially based on the algorithm
found in gdb/sim/arm/thumbemu.c.

I've backed out the silly 'T' suffix on PC in this version as requested
by RMK. At this point, I prefer to avoid making any of the other changes
discussed, e.g. dump_instr() for bad instruction cases, in order to get
the basic Thumb support change committed.

Thanks!

Signed-off-by: George G. Davis
Signed-off-by: Russell King

arch/arm/mm/alignment.c

@@ -3,6 +3,9 @@
  *
  *  Copyright (C) 1995  Linus Torvalds
  *  Modifications for ARM processor (c) 1995-2001 Russell King
+ *  Thumb aligment fault fixups (c) 2004 MontaVista Software, Inc.
+ *  - Adapted from gdb/sim/arm/thumbemu.c -- Thumb instruction emulation.
+ *    Copyright (C) 1996, Cygnus Software Technologies Ltd.
  *
  * 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
@@ -449,16 +452,146 @@ do_alignment_ldmstm(unsigned long addr, unsigned long instr, struct pt_regs *reg
 	return TYPE_ERROR;
 }
 
+/*
+ * Convert Thumb ld/st instruction forms to equivalent ARM instructions so
+ * we can reuse ARM userland alignment fault fixups for Thumb.
+ *
+ * This implementation was initially based on the algorithm found in
+ * gdb/sim/arm/thumbemu.c. It is basically just a code reduction of same
+ * to convert only Thumb ld/st instruction forms to equivalent ARM forms.
+ *
+ * NOTES:
+ * 1. Comments below refer to ARM ARM DDI0100E Thumb Instruction sections.
+ * 2. If for some reason we're passed an non-ld/st Thumb instruction to
+ *    decode, we return 0xdeadc0de. This should never happen under normal
+ *    circumstances but if it does, we've got other problems to deal with
+ *    elsewhere and we obviously can't fix those problems here.
+ */
+
+static unsigned long
+thumb2arm(u16 tinstr)
+{
+	u32 L = (tinstr & (1<<11)) >> 11;
+
+	switch ((tinstr & 0xf800) >> 11) {
+	/* 6.5.1 Format 1: */
+	case 0x6000 >> 11:				/* 7.1.52 STR(1) */
+	case 0x6800 >> 11:				/* 7.1.26 LDR(1) */
+	case 0x7000 >> 11:				/* 7.1.55 STRB(1) */
+	case 0x7800 >> 11:				/* 7.1.30 LDRB(1) */
+		return 0xe5800000 |
+			((tinstr & (1<<12)) << (22-12)) |	/* fixup */
+			(L<<20) |				/* L==1? */
+			((tinstr & (7<<0)) << (12-0)) |		/* Rd */
+			((tinstr & (7<<3)) << (16-3)) |		/* Rn */
+			((tinstr & (31<<6)) >>			/* immed_5 */
+				(6 - ((tinstr & (1<<12)) ? 0 : 2)));
+	case 0x8000 >> 11:				/* 7.1.57 STRH(1) */
+	case 0x8800 >> 11:				/* 7.1.32 LDRH(1) */
+		return 0xe1c000b0 |
+			(L<<20) |				/* L==1? */
+			((tinstr & (7<<0)) << (12-0)) |		/* Rd */
+			((tinstr & (7<<3)) << (16-3)) |		/* Rn */
+			((tinstr & (7<<6)) >> (6-1)) |	 /* immed_5[2:0] */
+			((tinstr & (3<<9)) >> (9-8));	 /* immed_5[4:3] */
+
+	/* 6.5.1 Format 2: */
+	case 0x5000 >> 11:
+	case 0x5800 >> 11:
+		{
+			static const u32 subset[8] = {
+				0xe7800000,		/* 7.1.53 STR(2) */
+				0xe18000b0,		/* 7.1.58 STRH(2) */
+				0xe7c00000,		/* 7.1.56 STRB(2) */
+				0xe19000d0,		/* 7.1.34 LDRSB */
+				0xe7900000,		/* 7.1.27 LDR(2) */
+				0xe19000b0,		/* 7.1.33 LDRH(2) */
+				0xe7d00000,		/* 7.1.31 LDRB(2) */
+				0xe19000f0		/* 7.1.35 LDRSH */
+			};
+			return subset[(tinstr & (7<<9)) >> 9] |
+			    ((tinstr & (7<<0)) << (12-0)) |	/* Rd */
+			    ((tinstr & (7<<3)) << (16-3)) |	/* Rn */
+			    ((tinstr & (7<<6)) >> (6-0));	/* Rm */
+		}
+
+	/* 6.5.1 Format 3: */
+	case 0x4800 >> 11:				/* 7.1.28 LDR(3) */
+		/* NOTE: This case is not technically possible. We're
+		 * 	 loading 32-bit memory data via PC relative
+		 *	 addressing mode. So we can and should eliminate
+		 *	 this case. But I'll leave it here for now.
+		 */
+		return 0xe59f0000 |
+		    ((tinstr & (7<<8)) << (12-8)) |		/* Rd */
+		    ((tinstr & 255) << (2-0));			/* immed_8 */
+
+	/* 6.5.1 Format 4: */
+	case 0x9000 >> 11:				/* 7.1.54 STR(3) */
+	case 0x9800 >> 11:				/* 7.1.29 LDR(4) */
+		return 0xe58d0000 |
+			(L<<20) |				/* L==1? */
+			((tinstr & (7<<8)) << (12-8)) |		/* Rd */
+			((tinstr & 255) << 2);			/* immed_8 */
+
+	/* 6.6.1 Format 1: */
+	case 0xc000 >> 11:				/* 7.1.51 STMIA */
+	case 0xc800 >> 11:				/* 7.1.25 LDMIA */
+		{
+			u32 Rn = (tinstr & (7<<8)) >> 8;
+			u32 W = ((L<<Rn) & (tinstr&255)) ? 0 : 1<<21;
+
+			return 0xe8800000 | W | (L<<20) | (Rn<<16) |
+				(tinstr&255);
+		}
+
+	/* 6.6.1 Format 2: */
+	case 0xb000 >> 11:				/* 7.1.48 PUSH */
+	case 0xb800 >> 11:				/* 7.1.47 POP */
+		if ((tinstr & (3 << 9)) == 0x0400) {
+			static const u32 subset[4] = {
+				0xe92d0000,	/* STMDB sp!,{registers} */
+				0xe92d4000,	/* STMDB sp!,{registers,lr} */
+				0xe8bd0000,	/* LDMIA sp!,{registers} */
+				0xe8bd8000	/* LDMIA sp!,{registers,pc} */
+			};
+			return subset[(L<<1) | ((tinstr & (1<<8)) >> 8)] |
+			    (tinstr & 255);		/* register_list */
+		}
+		/* Else fall through for illegal instruction case */
+
+	default:
+		return 0xdeadc0de;
+	}
+}
+
 static int
 do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 {
 	union offset_union offset;
-	unsigned long instr, instrptr;
+	unsigned long instr = 0, instrptr;
 	int (*handler)(unsigned long addr, unsigned long instr, struct pt_regs *regs);
 	unsigned int type;
+	mm_segment_t fs;
+	unsigned int fault;
+	u16 tinstr = 0;
 
 	instrptr = instruction_pointer(regs);
-	instr = *(unsigned long *)instrptr;
+
+	fs = get_fs();
+	set_fs(KERNEL_DS);
+	if thumb_mode(regs) {
+		fault = __get_user(tinstr, (u16 *)(instrptr & ~1));
+		if (!(fault))
+			instr = thumb2arm(tinstr);
+	} else
+		fault = __get_user(instr, (u32 *)instrptr);
+	set_fs(fs);
+
+	if (fault) {
+		type = TYPE_FAULT;
+ 		goto bad_or_fault;
+	}
 
 	if (user_mode(regs))
 		goto user;
@@ -467,7 +600,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
  fixup:
 
-	regs->ARM_pc += 4;
+	regs->ARM_pc += thumb_mode(regs) ? 2 : 4;
 
 	switch (CODING_BITS(instr)) {
 	case 0x00000000:	/* ldrh or strh */
@@ -537,7 +670,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
  bad_or_fault:
 	if (type == TYPE_ERROR)
 		goto bad;
-	regs->ARM_pc -= 4;
+	regs->ARM_pc -= thumb_mode(regs) ? 2 : 4;
 	/*
 	 * We got a fault - fix it up, or die.
 	 */
@@ -549,7 +682,9 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	 * Oops, we didn't handle the instruction.
 	 */
 	printk(KERN_ERR "Alignment trap: not handling instruction "
-		"%08lx at [<%08lx>]\n", instr, instrptr);
+		"%0*lx at [<%08lx>]\n",
+		thumb_mode(regs) ? 4 : 8,
+		thumb_mode(regs) ? tinstr : instr, instrptr);
 	ai_skipped += 1;
 	return 1;
 
@@ -557,9 +692,12 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	ai_user += 1;
 
 	if (ai_usermode & 1)
-		printk("Alignment trap: %s (%d) PC=0x%08lx Instr=0x%08lx "
+		printk("Alignment trap: %s (%d) PC=0x%08lx Instr=0x%0*lx "
 		       "Address=0x%08lx FSR 0x%03x\n", current->comm,
-			current->pid, instrptr, instr, addr, fsr);
+			current->pid, instrptr,
+		        thumb_mode(regs) ? 4 : 8,
+		        thumb_mode(regs) ? tinstr : instr,
+		        addr, fsr);
 
 	if (ai_usermode & 2)
 		goto fixup;