[Xen-devel] [PATCH v2 02/12] x86/alternatives: allow using assembler macros in favor of C ones

["Jan Beulich" <JBeulich@xxxxxxxx>]

As was validly pointed out as motivation for similar Linux side changes
(https://lkml.org/lkml/2018/6/22/677), using long sequences of
directives and auxiliary instructions, like is commonly the case when
setting up an alternative patch site, gcc can be mislead into believing
an asm() to be more heavy weight than it really is. By presenting it
with an assembler macro invocation instead, this can be avoided.

Initially I wanted to outright change the C macros ALTERNATIVE() and
ALTERNATIVE_2() to invoke the respective assembler ones, but doing so
would require quite a bit of cleanup of some use sites, because of the
exra necessary quoting combined with the need that each assembler macro
argument must consist of just a single string literal. We can consider
working towards that subsequently.

For now, set the stage of using the assembler macros here by providing a
new generated header, being the slightly massaged pre-processor output
of (for now just) alternative-asm.h. The massaging is primarily to be
able to properly track the build dependency: For this, we need the C
compiler to see the inclusion, which means we shouldn't directly use an
asm(". include ...") directive.

The dependency added to asm-offsets.s is not a true one; it's just the
easiest approach I could think of to make sure the new header gets
generated early on, without having to fiddle with xen/Makefile (and
introducing some x86-specific construct there).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Re-base.

--- a/.gitignore
+++ b/.gitignore
@@ -293,6 +293,7 @@ xen/.config.old
 xen/System.map
 xen/arch/arm/asm-offsets.s
 xen/arch/arm/xen.lds
+xen/arch/x86/asm-macros.i
 xen/arch/x86/asm-offsets.s
 xen/arch/x86/boot/mkelf32
 xen/arch/x86/xen.lds
--- a/xen/arch/x86/Makefile
+++ b/xen/arch/x86/Makefile
@@ -215,9 +215,22 @@ $(TARGET).efi: prelink-efi.o $(note_file
 efi/boot.init.o efi/runtime.o efi/compat.o efi/buildid.o: 
$(BASEDIR)/arch/x86/efi/built_in.o
 efi/boot.init.o efi/runtime.o efi/compat.o efi/buildid.o: ;
 
-asm-offsets.s: $(TARGET_SUBARCH)/asm-offsets.c
+asm-offsets.s: $(TARGET_SUBARCH)/asm-offsets.c 
$(BASEDIR)/include/asm-x86/asm-macros.h
        $(CC) $(filter-out -Wa$(comma)% -flto,$(CFLAGS)) -S -o $@ $<
 
+asm-macros.i: CFLAGS += -D__ASSEMBLY__ -P
+
+$(BASEDIR)/include/asm-x86/asm-macros.h: asm-macros.i Makefile
+       echo '#if 0' >$@.new
+       echo '.if 0' >>$@.new
+       echo '#endif' >>$@.new
+       echo 'asm ( ".include \"$@\"" );' >>$@.new
+       echo '#if 0' >>$@.new
+       echo '.endif' >>$@.new
+       cat $< >>$@.new
+       echo '#endif' >>$@.new
+       $(call move-if-changed,$@.new,$@)
+
 xen.lds: xen.lds.S
        $(CC) -P -E -Ui386 $(filter-out -Wa$(comma)%,$(AFLAGS)) -o $@ $<
        sed -e 's/xen\.lds\.o:/xen\.lds:/g' <.xen.lds.d >.xen.lds.d.new
@@ -237,6 +250,7 @@ efi/mkreloc: efi/mkreloc.c
 .PHONY: clean
 clean::
        rm -f asm-offsets.s *.lds boot/*.o boot/*~ boot/core boot/mkelf32
+       rm -f asm-macros.i $(BASEDIR)/include/asm-x86/asm-macros.*
        rm -f $(BASEDIR)/.xen-syms.[0-9]* boot/.*.d
        rm -f $(BASEDIR)/.xen.efi.[0-9]* efi/*.efi efi/mkreloc
        rm -f boot/cmdline.S boot/reloc.S boot/*.lnk boot/*.bin
--- /dev/null
+++ b/xen/arch/x86/asm-macros.c
@@ -0,0 +1 @@
+#include <asm/alternative-asm.h>
--- a/xen/include/asm-x86/alternative.h
+++ b/xen/include/asm-x86/alternative.h
@@ -1,11 +1,12 @@
 #ifndef __X86_ALTERNATIVE_H__
 #define __X86_ALTERNATIVE_H__
 
+#ifdef __ASSEMBLY__
 #include <asm/alternative-asm.h>
-
-#ifndef __ASSEMBLY__
+#else
 #include <xen/stringify.h>
 #include <xen/types.h>
+#include <asm/asm-macros.h>
 
 struct __packed alt_instr {
     int32_t  orig_offset;   /* original instruction */
@@ -26,14 +27,6 @@ extern void add_nops(void *insns, unsign
 extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
 extern void alternative_instructions(void);
 
-asm ( ".macro mknops nr_bytes\n\t"
-#ifdef HAVE_AS_NOPS_DIRECTIVE
-      ".nops \\nr_bytes, " __stringify(ASM_NOP_MAX) "\n\t"
-#else
-      ".skip \\nr_bytes, 0x90\n\t"
-#endif
-      ".endm\n\t" );
-
 #define alt_orig_len       "(.LXEN%=_orig_e - .LXEN%=_orig_s)"
 #define alt_pad_len        "(.LXEN%=_orig_p - .LXEN%=_orig_e)"
 #define alt_total_len      "(.LXEN%=_orig_p - .LXEN%=_orig_s)"




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