[PATCH v5 8/8] common: honor CONFIG_CC_SPLIT_SECTIONS also for assembly functions

[Jan Beulich <jbeulich@xxxxxxxx>]

Leverage the new infrastructure in xen/linkage.h to also switch to per-
function sections (when configured), deriving the specific name from the
"base" section in use at the time FUNC() is invoked.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
TBD: Since we use .subsection in UNLIKELY_START(), a perhaps not really
     wanted side effect of this change is that respective out-of-line
     code now moves much closer to its original (invoking) code.

TBD: Of course something with the same overall effect, but less
     impactful might do in Config.mk. E.g. $(filter-out -D%,$(3))
     instead of $(firstword (3)).

TBD: On top of Roger's respective patch (for livepatch), also respect
     CONFIG_FUNCTION_ALIGNMENT.

Note that we'd need to split DATA() in order to separate r/w and r/o
contributions. Further splitting might be needed to also support more
advanced attributes (e.g. merge), hence why this isn't done right here.
Sadly while a new section's name can be derived from the presently in
use, its attributes cannot be. Perhaps the only thing we can do is give
DATA() a 2nd mandatory parameter. Then again I guess most data
definitions could be moved to C anyway.
---
v5: Re-base over changes earlier in the series.
v4: Re-base.
v2: Make detection properly fail on old gas (by adjusting
    cc-option-add-closure).

--- a/Config.mk
+++ b/Config.mk
@@ -102,7 +102,7 @@ cc-option = $(shell if $(1) $(2:-Wno-%=-
 # Usage: $(call cc-option-add CFLAGS,CC,-march=winchip-c6)
 cc-option-add = $(eval $(call cc-option-add-closure,$(1),$(2),$(3)))
 define cc-option-add-closure
-    ifneq ($$(call cc-option,$$($(2)),$(3),n),n)
+    ifneq ($$(call cc-option,$$($(2)),$(firstword $(3)),n),n)
         $(1) += $(3)
     endif
 endef
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -409,6 +409,9 @@ AFLAGS += -D__ASSEMBLY__
 
 $(call cc-option-add,AFLAGS,CC,-Wa$$(comma)--noexecstack)
 
+# Check to see whether the assmbler supports the --sectname-subst option.
+$(call cc-option-add,AFLAGS,CC,-Wa$$(comma)--sectname-subst 
-DHAVE_AS_SECTNAME_SUBST)
+
 LDFLAGS-$(call ld-option,--warn-rwx-segments) += --no-warn-rwx-segments
 
 CFLAGS += $(CFLAGS-y)
--- a/xen/arch/arm/xen.lds.S
+++ b/xen/arch/arm/xen.lds.S
@@ -156,6 +156,9 @@ SECTIONS
   .init.text : {
        _sinittext = .;
        *(.init.text)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+       *(.init.text.*)
+#endif
        _einittext = .;
        . = ALIGN(PAGE_SIZE);        /* Avoid mapping alt insns executable */
        *(.altinstr_replacement)
--- a/xen/arch/ppc/xen.lds.S
+++ b/xen/arch/ppc/xen.lds.S
@@ -104,6 +104,9 @@ SECTIONS
     .init.text : {
         _sinittext = .;
         *(.init.text)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+        *(.init.text.*)
+#endif
         _einittext = .;
         . = ALIGN(PAGE_SIZE);        /* Avoid mapping alt insns executable */
     } :text
--- a/xen/arch/riscv/xen.lds.S
+++ b/xen/arch/riscv/xen.lds.S
@@ -104,6 +104,9 @@ SECTIONS
     .init.text : {
         _sinittext = .;
         *(.init.text)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+        *(.init.text.*)
+#endif
         _einittext = .;
         . = ALIGN(PAGE_SIZE);        /* Avoid mapping alt insns executable */
     } :text
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -86,6 +86,9 @@ SECTIONS
        . = ALIGN(PAGE_SIZE);
        _stextentry = .;
        *(.text.entry)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+       *(.text.entry.*)
+#endif
        . = ALIGN(PAGE_SIZE);
        _etextentry = .;
 
@@ -214,6 +217,9 @@ SECTIONS
 #endif
        _sinittext = .;
        *(.init.text)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+       *(.init.text.*)
+#endif
        *(.text.startup)
        _einittext = .;
        /*
--- a/xen/include/xen/linkage.h
+++ b/xen/include/xen/linkage.h
@@ -19,6 +19,14 @@
 
 #define SYM_ALIGN(align...) .balign align
 
+#if defined(HAVE_AS_SECTNAME_SUBST) && defined(CONFIG_CC_SPLIT_SECTIONS)
+# define SYM_PUSH_SECTION(name, attr) \
+         .pushsection %S.name, attr, %progbits; \
+         .equ .Lsplit_section, 1
+#else
+# define SYM_PUSH_SECTION(name, attr)
+#endif
+
 #define SYM_L_GLOBAL(name) .globl name; .hidden name
 #define SYM_L_WEAK(name)   .weak name
 #define SYM_L_LOCAL(name)  /* nothing */
@@ -33,7 +41,14 @@
         SYM_ALIGN(align);                         \
         name:
 
-#define END(name) .size name, . - name
+#define END(name) \
+        .size name, . - name; \
+        .ifdef .Lsplit_section; \
+            .if .Lsplit_section; \
+                .popsection; \
+                .equ .Lsplit_section, 0; \
+            .endif; \
+        .endif
 
 /*
  * CODE_FILL in particular may need to expand to nothing (e.g. for RISC-V), in
@@ -47,6 +62,7 @@
 #endif
 
 #define FUNC(name, align...) \
+        SYM_PUSH_SECTION(name, "ax"); \
         SYM(name, FUNC, GLOBAL, DO_CODE_ALIGN(align))
 #define LABEL(name, align...) \
         SYM(name, NONE, GLOBAL, DO_CODE_ALIGN(align))
@@ -54,6 +70,7 @@
         SYM(name, DATA, GLOBAL, LASTARG(DATA_ALIGN, ## align), DATA_FILL)
 
 #define FUNC_LOCAL(name, align...) \
+        SYM_PUSH_SECTION(name, "ax"); \
         SYM(name, FUNC, LOCAL, DO_CODE_ALIGN(align))
 #define LABEL_LOCAL(name, align...) \
         SYM(name, NONE, LOCAL, DO_CODE_ALIGN(align))