powerpc: Introduce infrastructure for feature sections with alternatives

The current feature section logic only supports nop'ing out code, this means
if you want to choose at runtime between instruction sequences, one or both
cases will have to execute the nop'ed out contents of the other section, eg:

BEGIN_FTR_SECTION
	or	1,1,1
END_FTR_SECTION_IFSET(FOO)
BEGIN_FTR_SECTION
	or	2,2,2
END_FTR_SECTION_IFCLR(FOO)

and the resulting code will be either,

	or	1,1,1
	nop

or,
	nop
	or	2,2,2

For small code segments this is fine, but for larger code blocks and in
performance criticial code segments, it would be nice to avoid the nops.
This commit starts to implement logic to allow the following:

BEGIN_FTR_SECTION
	or	1,1,1
FTR_SECTION_ELSE
	or	2,2,2
ALT_FTR_SECTION_END_IFSET(FOO)

and the resulting code will be:

	or	1,1,1
or,
	or	2,2,2

We achieve this by extending the existing FTR macros. The current feature
section semantic just becomes a special case, ie. if the else case is empty
we nop out the default case.

The key limitation is that the size of the else case must be less than or
equal to the size of the default case. If the else case is smaller the
remainder of the section is nop'ed.

We let the linker put the else case code in with the rest of the text,
so that relative branches from the else case are more likley to link,
this has the disadvantage that we can't free the unused else cases.

This commit introduces the required macro and linker script changes, but
does not enable the patching of the alternative sections.

We also need to update two hand-made section entries in reg.h and timex.h

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

4 files changed, 5 insertions, 3 deletions

diff --git a/arch/powerpc/kernel/vdso32/vdso32.lds.S b/arch/powerpc/kernel/vdso32/vdso32.lds.S
index 9352ab5..2717935 100644
--- a/arch/powerpc/kernel/vdso32/vdso32.lds.S
+++ b/arch/powerpc/kernel/vdso32/vdso32.lds.S
@@ -24,7 +24,7 @@ SECTIONS
 
 	. = ALIGN(16);
 	.text		: {
-		*(.text .stub .text.* .gnu.linkonce.t.*)
+		*(.text .stub .text.* .gnu.linkonce.t.* __ftr_alt_*)
 	}
 	PROVIDE(__etext = .);
 	PROVIDE(_etext = .);
diff --git a/arch/powerpc/kernel/vdso64/vdso64.lds.S b/arch/powerpc/kernel/vdso64/vdso64.lds.S
index 7d6585f..e608d1b 100644
--- a/arch/powerpc/kernel/vdso64/vdso64.lds.S
+++ b/arch/powerpc/kernel/vdso64/vdso64.lds.S
@@ -24,7 +24,7 @@ SECTIONS
 
 	. = ALIGN(16);
 	.text		: {
-		*(.text .stub .text.* .gnu.linkonce.t.*)
+		*(.text .stub .text.* .gnu.linkonce.t.* __ftr_alt_*)
 		*(.sfpr .glink)
 	}						:text
 	PROVIDE(__etext = .);
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 0c3000b..3c07811 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -35,7 +35,7 @@ SECTIONS
 		ALIGN_FUNCTION();
 		*(.text.head)
 		_text = .;
-		*(.text .fixup .text.init.refok .exit.text.refok)
+		*(.text .fixup .text.init.refok .exit.text.refok __ftr_alt_*)
 		SCHED_TEXT
 		LOCK_TEXT
 		KPROBES_TEXT
diff --git a/arch/powerpc/lib/feature-fixups.c b/arch/powerpc/lib/feature-fixups.c
index f6fd5d2..973d547e 100644
--- a/arch/powerpc/lib/feature-fixups.c
+++ b/arch/powerpc/lib/feature-fixups.c
@@ -22,6 +22,8 @@ struct fixup_entry {
 	unsigned long	value;
 	long		start_off;
 	long		end_off;
+	long		alt_start_off;
+	long		alt_end_off;
 };
 
 static void patch_feature_section(unsigned long value, struct fixup_entry *fcur)