[v8,RESEND,6/6] common: honor CONFIG_CC_SPLIT_SECTIONS also for assembly functions

Commit Message

Jan Beulich March 13, 2025, 8:10 a.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]>
Tested-by: Luca Fancellu <[e-mail]> # arm
---
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).

Note that we'd need to split DATA() in order to separate r/w, r/o, and
BSS 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.

An alternative to the "override" in arm64/head.S would be to use
LABEL{,_LOCAL}() instead of FUNC{,_LOCAL}() there. Yet that would also
lose the type information then. Question is whether the annotated ranges
really are "functions" in whichever wide or narrow sense.
---
v7: Override SYM_PUSH_SECTION() in arch/x86/indirect-thunk.S. Re-base,
    notably to deal with fallout from fba250ae604e ("xen/arm64: head:
    Add missing code symbol annotations").
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).

Patch

--- 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
@@ -415,6 +415,9 @@  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/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -28,6 +28,14 @@ 
 #include <asm/arm64/efibind.h>
 #endif
 
+/*
+ * Code here is, at least in part, ordering sensitive.  Annotations
+ * from xen/linkage.h therefore may not switch sections (honoring
+ * CONFIG_CC_SPLIT_SECTIONS).  Override the respective macro.
+ */
+#undef SYM_PUSH_SECTION
+#define SYM_PUSH_SECTION(name, attr)
+
 #define __HEAD_FLAG_PAGE_SIZE   ((PAGE_SHIFT - 10) / 2)
 
 #define __HEAD_FLAG_PHYS_BASE   1
--- a/xen/arch/arm/xen.lds.S
+++ b/xen/arch/arm/xen.lds.S
@@ -140,6 +140,9 @@  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
@@ -103,6 +103,9 @@  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
@@ -98,6 +98,9 @@  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/indirect-thunk.S
+++ b/xen/arch/x86/indirect-thunk.S
@@ -11,6 +11,10 @@ 
 
 #include <asm/asm_defns.h>
 
+/* Section placement is done explicitly here; override the respective macro. */
+#undef SYM_PUSH_SECTION
+#define SYM_PUSH_SECTION(name, attr)
+
 .macro IND_THUNK_RETPOLINE reg:req
         call 1f
         int3
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -31,6 +31,9 @@  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
@@ -771,6 +771,9 @@  END(common_interrupt)
 FUNC(entry_PF)
         ENDBR64
         movb  $X86_EXC_PF, EFRAME_entry_vector(%rsp)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+        jmp   handle_exception
+#endif
 END(entry_PF)
 /* No special register assumptions. */
 FUNC(handle_exception, 0)
@@ -1084,8 +1087,11 @@  FUNC(entry_NMI)
         ENDBR64
         pushq $0
         movb  $X86_EXC_NMI, EFRAME_entry_vector(%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
@@ -85,6 +85,9 @@  SECTIONS
        . = ALIGN(PAGE_SIZE);
        _stextentry = .;
        *(.text.entry)
+#ifdef CONFIG_CC_SPLIT_SECTIONS
+       *(.text.entry.*)
+#endif
        . = ALIGN(PAGE_SIZE);
        _etextentry = .;
 
@@ -204,6 +207,9 @@  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
@@ -18,6 +18,14 @@ 
 
 #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 */
@@ -32,7 +40,14 @@ 
         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))