[PATCH] x86/fault: Make show_error_code() more extensible and tweak its formatting

[Sean Christopherson]

  - Initialize each bit individually in the error_code_chars array.  This
    allows for non-contiguous bits and is self-documenting.  Define a
    macro to make the initialization code somewhatmore readable.

  - Reverse the decode order so it's consistent with the raw display.

  - Use ARRAY_SIZE instead of hardcoding '6' in multiple locations.

  - Use '!' for the negative case to better contrast against '+'.

  - Display four digits (was two) when printing the raw error code.

  - Remove @regs from show_error_code().

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/mm/fault.c | 47 ++++++++++++++++++++++++---------------------
 1 file changed, 25 insertions(+), 22 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 23dc7163f6ac..cd28d058ed39 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -605,45 +605,48 @@ static void show_ldttss(const struct desc_ptr *gdt, const char *name, u16 index)
 
 /*
  * This maps the somewhat obscure error_code number to symbolic text:
- *
- * P = protection fault   (X86_PF_PROT)
- * W = write access       (X86_PF_WRITE)
- * U = user-mode access   (X86_PF_USER)
- * S = supervisor mode    (X86_PF_RSVD)
- * I = instruction fault  (X86_PF_INSTR)
- * K = keys fault         (X86_PF_PK)
  */
-static const char error_code_chars[] = "PWUSIK";
+#define EC(x) ilog2(X86_PF_##x)
+static const char error_code_chars[] = {
+	[EC(PROT)]	= 'P',
+	[EC(WRITE)]	= 'W',
+	[EC(USER)]	= 'U',
+	[EC(RSVD)]	= 'S',
+	[EC(INSTR)]	= 'I',
+	[EC(PK)]	= 'K',
+};
 
 /*
- * This helper function transforms the #PF error_code bits into " +P -W +U -R -I -K"
+ * This helper function transforms the #PF error_code bits into " +P !W +U !R !I !K"
  * type of descriptive, almost human-readable error strings:
  */
-static void show_error_code(struct pt_regs *regs, unsigned long error_code)
+static void show_error_code(unsigned long error_code)
 {
-	unsigned int bit, mask;
-	char err_txt[6*3+1]; /* Fixed length of 6 bits decoded plus zero at the end */
+	char err_txt[ARRAY_SIZE(error_code_chars)*3+1]; /* 3 chars per bit plus zero at the end */
+	unsigned offset = 0;
+	unsigned long mask;
+	int i;
 
-	/* We go from the X86_PF_PROT bit to the X86_PF_PK bit: */
-
-	for (bit = 0; bit < 6; bit++) {
-		unsigned int offset = bit*3;
+	for (i = ARRAY_SIZE(error_code_chars) - 1; i >= 0; i--) {
+		if (!error_code_chars[i])
+			continue;
 
 		err_txt[offset+0] = ' ';
 
-		mask = 1 << bit;
+		mask = 1 << i;
 		if (error_code & mask)
 			err_txt[offset+1] = '+';
 		else
-			err_txt[offset+1] = '-';
+			err_txt[offset+1] = '!';
 
-		err_txt[offset+2] = error_code_chars[bit];
+		err_txt[offset+2] = error_code_chars[i];
+		offset += 3;
 	}
 
 	/* Close the string: */
-	err_txt[sizeof(err_txt)-1] = 0;
+	err_txt[offset] = 0;
 
-	pr_alert("#PF error code: %s (%02lx)\n", err_txt, error_code);
+	pr_alert("#PF error code(%04lx): %s\n", error_code, err_txt);
 }
 
 static void
@@ -676,7 +679,7 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
 		 address < PAGE_SIZE ? "NULL pointer dereference" : "paging request",
 		 (void *)address);
 
-	show_error_code(regs, error_code);
+	show_error_code(error_code);
 
 	if (!(error_code & X86_PF_USER) && user_mode(regs)) {
 		struct desc_ptr idt, gdt;
-- 
2.19.2