[v3,9/8] common: honor CONFIG_CC_SPLIT_SECTIONS also for assembly functions

Commit Message

Jan Beulich July 17, 2023, 2:18 p.m. UTC

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 <[e-mail]>
---
RFC: 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.

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. Hence 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.

Comments

Jan Beulich July 18, 2023, 12:28 p.m. UTC | #1

On 17.07.2023 16:18, Jan Beulich wrote:
> 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 <[e-mail]>

This additional change

--- a/Config.mk
+++ b/Config.mk
@@ -115,7 +115,7 @@ cc-option = $(shell if test -z "`echo 'v
 # 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

is needed for ...

> --- a/xen/Makefile
> +++ b/xen/Makefile
> @@ -399,6 +399,9 @@ 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)

... the pair of options passed in one go here to work (when old
gas is in use). Of course something with the same overall effect,
but less impactful might do. E.g. $(filter-out -D%,$(3)) instead
of $(firstword (3)). Thoughts anyone?

Jan

Patch

--- a/xen/Makefile
+++ b/xen/Makefile
@@ -399,6 +399,9 @@  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
@@ -154,6 +154,9 @@  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
@@ -106,6 +106,9 @@  SECTIONS
     DECL_SECTION(.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
@@ -92,6 +92,9 @@  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
@@ -21,6 +21,14 @@ 
 
 #define SYM_ALIGN(algn...) .balign algn
 
+#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 */
@@ -35,7 +43,14 @@ 
         SYM_ALIGN(algn);                          \
         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
@@ -49,6 +64,7 @@ 
 #endif
 
 #define FUNC(name, algn...) \
+        SYM_PUSH_SECTION(name, "ax"); \
         SYM(name, FUNC, GLOBAL, DO_CODE_ALIGN(algn))
 #define LABEL(name, algn...) \
         SYM(name, NONE, GLOBAL, DO_CODE_ALIGN(algn))
@@ -56,6 +72,7 @@ 
         SYM(name, DATA, GLOBAL, LASTARG(DATA_ALIGN, ## algn), DATA_FILL)
 
 #define FUNC_LOCAL(name, algn...) \
+        SYM_PUSH_SECTION(name, "ax"); \
         SYM(name, FUNC, LOCAL, DO_CODE_ALIGN(algn))
 #define LABEL_LOCAL(name, algn...) \
         SYM(name, NONE, LOCAL, DO_CODE_ALIGN(algn))