[RFC][PATCH 21/22] x86,word-at-a-time: Remove .fixup usage

[Peter Zijlstra]

Push the load_unaligned_zeropad() exception into exception context by
adding a new extable type. This however requires we have both the
address and the output register. Since we can only have a single
register argument, use the same for both.

This also means the source can no longer use "m" constraint.

XXX: I'm not really happy with this patch

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/extable_fixup_types.h |    2 ++
 arch/x86/include/asm/word-at-a-time.h      |   27 +++++++--------------------
 arch/x86/mm/extable.c                      |   17 +++++++++++++++++
 3 files changed, 26 insertions(+), 20 deletions(-)

--- a/arch/x86/include/asm/extable_fixup_types.h
+++ b/arch/x86/include/asm/extable_fixup_types.h
@@ -47,4 +47,6 @@
 #define	EX_TYPE_UACCESS_LEN4		(EX_TYPE_UACCESS_LEN | EX_TYPE_IMM(4))
 #define	EX_TYPE_UACCESS_LEN8		(EX_TYPE_UACCESS_LEN | EX_TYPE_IMM(8))
 
+#define	EX_TYPE_LOAD_UNALIGNED		21 /* reg := (reg) */
+
 #endif
--- a/arch/x86/include/asm/word-at-a-time.h
+++ b/arch/x86/include/asm/word-at-a-time.h
@@ -79,27 +79,14 @@ static inline unsigned long find_zero(un
  */
 static inline unsigned long load_unaligned_zeropad(const void *addr)
 {
-	unsigned long ret, dummy;
+	unsigned long ret;
+
+	asm("1:\tmov (%0),%0\n"
+	    "2:\n"
+	    _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_LOAD_UNALIGNED, %0)
+	    : "=&r" (ret)
+	    : "0" ((unsigned long)addr));
 
-	asm(
-		"1:\tmov %2,%0\n"
-		"2:\n"
-		".section .fixup,\"ax\"\n"
-		"3:\t"
-		"lea %2,%1\n\t"
-		"and %3,%1\n\t"
-		"mov (%1),%0\n\t"
-		"leal %2,%%ecx\n\t"
-		"andl %4,%%ecx\n\t"
-		"shll $3,%%ecx\n\t"
-		"shr %%cl,%0\n\t"
-		"jmp 2b\n"
-		".previous\n"
-		_ASM_EXTABLE(1b, 3b)
-		:"=&r" (ret),"=&c" (dummy)
-		:"m" (*(unsigned long *)addr),
-		 "i" (-sizeof(unsigned long)),
-		 "i" (sizeof(unsigned long)-1));
 	return ret;
 }
 
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -170,6 +170,21 @@ static bool ex_handler_uaccess_len(const
 	return ex_handler_uaccess(fixup, regs, trapnr);
 }
 
+static bool ex_handler_load_unaligned_zeropad(const struct exception_table_entry *fixup,
+					      struct pt_regs *regs, int reg)
+{
+	unsigned long addr, offset, data;
+
+	addr = *pt_regs_nr(regs, reg);
+	offset = addr & (sizeof(unsigned long) - 1);
+	addr &= ~(sizeof(unsigned long) - 1);
+	data = *(unsigned long *)addr;
+	data >>= offset*8;
+	*pt_regs_nr(regs, reg) = data;
+
+	return ex_handler_default(fixup, regs);
+}
+
 #define EX_TYPE_MASK	0x000000FF
 #define EX_REG_MASK	0x00000F00
 #define EX_FLAG_MASK	0x0000F000
@@ -251,6 +266,8 @@ int fixup_exception(struct pt_regs *regs
 		return ex_handler_kvm_fastop(e, regs);
 	case EX_TYPE_UACCESS_LEN:
 		return ex_handler_uaccess_len(e, regs, trapnr, reg, imm);
+	case EX_TYPE_LOAD_UNALIGNED:
+		return ex_handler_load_unaligned_zeropad(e, regs, reg);
 	}
 	BUG();
 }