Re: [PATCH] ftrace: Allow section alignment

["Matt Fleming" <mjf@netbsd.org>]

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2008/11/7 Steven Rostedt <rostedt@goodmis.org>:
>
> On Fri, 7 Nov 2008, Peter Zijlstra wrote:
>
>> On Fri, 2008-11-07 at 13:12 +0000, Matt Fleming wrote:
>> > Provide a means of aligning the __mcount_loc section.
>>
>> Not that I object to the patch, but this changelog needs work.
>>
>> Its wrong, it doesn't provide means, it plain does.
>> It doesn't tell us why.
>
> Also, please tell us what broke. Did you have a compiler that did
> not align it properly?  As long as the alignment is less than or equal to
> the word size, it is fine. If the alignment is larger than the word size,
> then there will be holes in the mcount_loc section and that will break
> ftrace. Heck, an alignment of 1 should work for everyone ;-) All the
> sections are packed together in an aligned space when it is pulled in by
> the linker. If that is not true, then the linker script needs to be fixed.
>

I've tried to explain the bug better in the attached patch. The issue
is not that there were gaps in __mcount_loc, rather that the addresses
in __mcount_loc were not aligned on a 4-byte boundary by the
assembler. Unaligned accesses are not supported by our architecture
and an alignment of 1-byte will likely lead to performance loss on
architectures that do support them.



>From 4161c8b2a1e5c93146fc6e9a638c018a14648c6b Mon Sep 17 00:00:00 2001
From: Matt Fleming <matthew.fleming@imgtec.com>
Date: Fri, 7 Nov 2008 13:26:25 +0000
Subject: [PATCH] ftrace: Align __mcount_loc sections

Align the __mcount_loc sections so that architectures with strict
alignment requirements need not worry about performing unaligned
accesses.

This fixes an issue where I was seeing unaligned accesses, which are not
supported on our architecture (the results of an unaligned access are
undefined).

Signed-off-by: Matt Fleming <matthew.fleming@imgtec.com>
---
 scripts/recordmcount.pl |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
index 6b9fe3e..2d0bfa1 100755
--- a/scripts/recordmcount.pl
+++ b/scripts/recordmcount.pl
@@ -134,6 +134,7 @@ my $section_regex;  # Find the start of a section
 my $function_regex;    # Find the name of a function
                        #    (return offset and func name)
 my $mcount_regex;      # Find the call site to mcount (return offset)
+my $alignment;         # The .align value to use for $mcount_section

 if ($arch eq "x86") {
     if ($bits == 64) {
@@ -148,6 +149,7 @@ if ($arch eq "x86_64") {
     $function_regex = "^([0-9a-fA-F]+)\\s+<(.*?)>:";
     $mcount_regex = "^\\s*([0-9a-fA-F]+):.*\\smcount([+-]0x[0-9a-zA-Z]+)?\$";
     $type = ".quad";
+    $alignment = 8;

     # force flags for this arch
     $ld .= " -m elf_x86_64";
@@ -160,6 +162,7 @@ if ($arch eq "x86_64") {
     $function_regex = "^([0-9a-fA-F]+)\\s+<(.*?)>:";
     $mcount_regex = "^\\s*([0-9a-fA-F]+):.*\\smcount\$";
     $type = ".long";
+    $alignment = 4;

     # force flags for this arch
     $ld .= " -m elf_i386";
@@ -288,6 +291,7 @@ sub update_funcs
            open(FILE, ">$mcount_s") || die "can't create $mcount_s\n";
            $opened = 1;
            print FILE "\t.section $mcount_section,\"a\",\@progbits\n";
+           print FILE "\t.align $alignment\n";
        }
        printf FILE "\t%s %s + %d\n", $type, $ref_func, $offsets[$i] - $offset;
     }
--
1.5.6.4

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From 4161c8b2a1e5c93146fc6e9a638c018a14648c6b Mon Sep 17 00:00:00 2001
From: Matt Fleming <matthew.fleming@imgtec.com>
Date: Fri, 7 Nov 2008 13:26:25 +0000
Subject: [PATCH] ftrace: Align __mcount_loc sections

Align the __mcount_loc sections so that architectures with strict
alignment requirements need not worry about performing unaligned
accesses.

This fixes an issue where I was seeing unaligned accesses, which are not
supported on our architecture (the results of an unaligned access are
undefined).

Signed-off-by: Matt Fleming <matthew.fleming@imgtec.com>
---
 scripts/recordmcount.pl |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
index 6b9fe3e..2d0bfa1 100755
--- a/scripts/recordmcount.pl
+++ b/scripts/recordmcount.pl
@@ -134,6 +134,7 @@ my $section_regex;	# Find the start of a section
 my $function_regex;	# Find the name of a function
 			#    (return offset and func name)
 my $mcount_regex;	# Find the call site to mcount (return offset)
+my $alignment;		# The .align value to use for $mcount_section
 
 if ($arch eq "x86") {
     if ($bits == 64) {
@@ -148,6 +149,7 @@ if ($arch eq "x86_64") {
     $function_regex = "^([0-9a-fA-F]+)\\s+<(.*?)>:";
     $mcount_regex = "^\\s*([0-9a-fA-F]+):.*\\smcount([+-]0x[0-9a-zA-Z]+)?\$";
     $type = ".quad";
+    $alignment = 8;
 
     # force flags for this arch
     $ld .= " -m elf_x86_64";
@@ -160,6 +162,7 @@ if ($arch eq "x86_64") {
     $function_regex = "^([0-9a-fA-F]+)\\s+<(.*?)>:";
     $mcount_regex = "^\\s*([0-9a-fA-F]+):.*\\smcount\$";
     $type = ".long";
+    $alignment = 4;
 
     # force flags for this arch
     $ld .= " -m elf_i386";
@@ -288,6 +291,7 @@ sub update_funcs
 	    open(FILE, ">$mcount_s") || die "can't create $mcount_s\n";
 	    $opened = 1;
 	    print FILE "\t.section $mcount_section,\"a\",\@progbits\n";
+	    print FILE "\t.align $alignment\n";
 	}
 	printf FILE "\t%s %s + %d\n", $type, $ref_func, $offsets[$i] - $offset;
     }
-- 
1.5.6.4