[PATCH v6 1/7] common: honor CONFIG_CC_SPLIT_SECTIONS also for assembly functions

[Jan Beulich <jbeulich@suse.com>]

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@suse.com>
---
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)). In fact Roger wants the detection to
     be in Kconfig, for LIVEPATCH to depend on it. Yet the whole
     underlying discussion there imo would first need settling (and
     therefore reviving).

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.
---
v6: Deal with x86'es entry_PF() and entry_int82() falling through to the
    next "function". Re-base.
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 assembler 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
@@ -157,6 +157,9 @@ 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/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -29,6 +29,9 @@ FUNC(entry_int82)
 
         mov   %rsp, %rdi
         call  do_entry_int82
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+        jmp   compat_test_all_events
+#endif
 END(entry_int82)
 
 /* %rbx: struct vcpu */
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -723,6 +723,9 @@ END(common_interrupt)
 FUNC(entry_PF)
         ENDBR64
         movl  $X86_EXC_PF, 4(%rsp)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+        jmp   handle_exception
+#endif
 END(entry_PF)
 /* No special register assumptions. */
 FUNC(handle_exception, 0)
@@ -1023,8 +1026,11 @@ FUNC(entry_NMI)
         ENDBR64
         pushq $0
         movl  $X86_EXC_NMI, 4(%rsp)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+        jmp   handle_ist_exception
+#endif
 END(entry_NMI)
-
+/* No special register assumptions. */
 FUNC(handle_ist_exception)
         ALTERNATIVE "", clac, X86_FEATURE_XEN_SMAP
         SAVE_ALL
--- 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))