[Xen-devel] [PATCH v2] x86/build: Use new .nop directive when available

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Newer versions of binutils are capable of emitting an exact number bytes worth
of optimised nops.  Use this in preference to .skip when available.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>

RFC until support is actually committed to binutils mainline.
---
 xen/arch/x86/Rules.mk                 |  4 ++++
 xen/include/asm-x86/alternative-asm.h | 14 ++++++++++++--
 xen/include/asm-x86/alternative.h     | 13 ++++++++++---
 3 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index e169d67..bf5047f 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -28,6 +28,10 @@ $(call as-option-add,CFLAGS,CC,".equ \"x\"$$(comma)1", \
 $(call as-option-add,CFLAGS,CC,\
     ".if ((1 > 0) < 0); .error \"\";.endif",,-DHAVE_AS_NEGATIVE_TRUE)
 
+# Check to see whether the assmbler supports the .nop directive.
+$(call as-option-add,CFLAGS,CC,\
+    ".L1: .L2: .nop (.L2 - .L1)$$(comma)9",-DHAVE_AS_NOP_DIRECTIVE)
+
 CFLAGS += -mno-red-zone -fpic -fno-asynchronous-unwind-tables
 
 # Xen doesn't use SSE interally.  If the compiler supports it, also skip the
diff --git a/xen/include/asm-x86/alternative-asm.h 
b/xen/include/asm-x86/alternative-asm.h
index 25f79fe..9e46bed 100644
--- a/xen/include/asm-x86/alternative-asm.h
+++ b/xen/include/asm-x86/alternative-asm.h
@@ -1,6 +1,8 @@
 #ifndef _ASM_X86_ALTERNATIVE_ASM_H_
 #define _ASM_X86_ALTERNATIVE_ASM_H_
 
+#include <asm/nops.h>
+
 #ifdef __ASSEMBLY__
 
 /*
@@ -18,6 +20,14 @@
     .byte \pad_len
 .endm
 
+.macro mknops nr_bytes
+#ifdef HAVE_AS_NOP_DIRECTIVE
+    .nop \nr_bytes, ASM_NOP_MAX
+#else
+    .skip \nr_bytes, 0x90
+#endif
+.endm
+
 #define orig_len               (.L\@_orig_e       -     .L\@_orig_s)
 #define pad_len                (.L\@_orig_p       -     .L\@_orig_e)
 #define total_len              (.L\@_orig_p       -     .L\@_orig_s)
@@ -43,7 +53,7 @@
      */
     .L\@_diff = repl_len(1) - orig_len
 
-    .skip as_true(.L\@_diff > 0) * .L\@_diff, 0x90
+     mknops (as_true(.L\@_diff > 0) * .L\@_diff)
 .L\@_orig_p:
 
     .pushsection .altinstructions, "a", @progbits
@@ -76,7 +86,7 @@
      */
     .L\@_diff = as_max(repl_len(1), repl_len(2)) - orig_len
 
-     .skip as_true(.L\@_diff > 0) * .L\@_diff, 0x90
+     mknops (as_true(.L\@_diff > 0) * .L\@_diff)
 .L\@_orig_p:
 
     .pushsection .altinstructions, "a", @progbits
diff --git a/xen/include/asm-x86/alternative.h 
b/xen/include/asm-x86/alternative.h
index d53cea0..51538b6 100644
--- a/xen/include/asm-x86/alternative.h
+++ b/xen/include/asm-x86/alternative.h
@@ -2,7 +2,6 @@
 #define __X86_ALTERNATIVE_H__
 
 #include <asm/alternative-asm.h>
-#include <asm/nops.h>
 
 #ifndef __ASSEMBLY__
 #include <xen/stringify.h>
@@ -27,6 +26,14 @@ extern void apply_alternatives(const struct alt_instr *start,
                                const struct alt_instr *end);
 extern void alternative_instructions(void);
 
+asm ( ".macro mknops nr_bytes\n\t"
+#ifdef HAVE_AS_NOP_DIRECTIVE
+      ".nop \\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)"
@@ -46,7 +53,7 @@ extern void alternative_instructions(void);
 #define OLDINSTR_1(oldinstr, n1)                                 \
     ".LXEN%=_orig_s:\n\t" oldinstr "\n .LXEN%=_orig_e:\n\t"      \
     ".LXEN%=_diff = "alt_repl_len(n1)"-"alt_orig_len"\n\t"       \
-    ".skip "AS_TRUE"(.LXEN%=_diff > 0) * .LXEN%=_diff, 0x90\n\t" \
+    "mknops ("AS_TRUE"(.LXEN%=_diff > 0) * .LXEN%=_diff)\n\t"    \
     ".LXEN%=_orig_p:\n\t"
 
 #define ALT_PADDING_LEN(n1, n2) \
@@ -55,7 +62,7 @@ extern void alternative_instructions(void);
 #define OLDINSTR_2(oldinstr, n1, n2)                             \
     ".LXEN%=_orig_s:\n\t" oldinstr "\n .LXEN%=_orig_e:\n\t"      \
     ".LXEN%=_diff = "ALT_PADDING_LEN(n1, n2)"\n\t"               \
-    ".skip "AS_TRUE"(.LXEN%=_diff > 0) * .LXEN%=_diff, 0x90\n\t" \
+    "mknops ("AS_TRUE"(.LXEN%=_diff > 0) * .LXEN%=_diff)\n\t"    \
     ".LXEN%=_orig_p:\n\t"
 
 #define ALTINSTR_ENTRY(feature, num)                                    \
-- 
2.1.4


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