[Xen-devel] [PATCH 7/7] 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                 |  1 +
 xen/include/asm-x86/alternative-asm.h | 16 +++++++++++++---
 xen/include/asm-x86/alternative.h     | 17 ++++++++++++-----
 3 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index 56b2ea8..c3b726c 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -20,6 +20,7 @@ $(call as-insn-check,CFLAGS,CC,"invept 
(%rax)$$(comma)%rax",-DHAVE_GAS_EPT)
 $(call as-insn-check,CFLAGS,CC,"rdrand %eax",-DHAVE_GAS_RDRAND)
 $(call as-insn-check,CFLAGS,CC,"rdfsbase %rax",-DHAVE_GAS_FSGSBASE)
 $(call as-insn-check,CFLAGS,CC,"rdseed %eax",-DHAVE_GAS_RDSEED)
+$(call as-insn-check,CFLAGS,CC,".nop 0$$(comma)9",-DHAVE_GAS_LONG_NOPS)
 $(call as-insn-check,CFLAGS,CC,".equ \"x\"$$(comma)1", \
                      -U__OBJECT_LABEL__ -DHAVE_GAS_QUOTED_SYM \
                      '-D__OBJECT_LABEL__=$(subst $(BASEDIR)/,,$(CURDIR))/$$@')
diff --git a/xen/include/asm-x86/alternative-asm.h 
b/xen/include/asm-x86/alternative-asm.h
index f7e37cb..a4893e7 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_GAS_LONG_NOPS
+    .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)
@@ -29,7 +39,7 @@
 .L\@_orig_s:
     \oldinstr
 .L\@_orig_e:
-     .skip (-((repl_len(1) - orig_len) > 0) * (repl_len(1) - orig_len)), 0x90
+     mknops (-((repl_len(1) - orig_len) > 0) * (repl_len(1) - orig_len))
 .L\@_orig_p:
 
     .pushsection .altinstructions, "a", @progbits
@@ -56,8 +66,8 @@
 .L\@_orig_s:
     \oldinstr
 .L\@_orig_e:
-    .skip (-((gas_max(repl_len(1), repl_len(2)) - orig_len) > 0) * \
-             (gas_max(repl_len(1), repl_len(2)) - orig_len)), 0x90
+    mknops (-((gas_max(repl_len(1), repl_len(2)) - orig_len) > 0) * \
+              (gas_max(repl_len(1), repl_len(2)) - orig_len))
 .L\@_orig_p:
 
     .pushsection .altinstructions, "a", @progbits
diff --git a/xen/include/asm-x86/alternative.h 
b/xen/include/asm-x86/alternative.h
index 20dea22..076d700 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_GAS_LONG_NOPS
+      ".nop \\nr_bytes, " __stringify(ASM_NOP_MAX) "\n\t"
+#else
+      ".skip \\nr_bytes, 0x90\n\t"
+#endif
+      ".endm\n\t" );
+
 #define repl_s(num)             ".L%=_repl_s"#num
 #define repl_e(num)             ".L%=_repl_e"#num
 
@@ -39,8 +46,8 @@ extern void alternative_instructions(void);
 
 #define OLDINSTR_1(oldinstr, n1)                              \
     ".L%=_orig_s:\n\t" oldinstr "\n .L%=_orig_e:\n\t"         \
-    ".skip (-(("alt_repl_len(n1)"-"alt_orig_len") > 0) * "    \
-             "("alt_repl_len(n1)"-"alt_orig_len")), 0x90\n\t" \
+    "mknops (-(("alt_repl_len(n1)"-"alt_orig_len") > 0) * "   \
+              "("alt_repl_len(n1)"-"alt_orig_len"))\n\t"      \
     ".L%=_orig_p:\n\t"
 
 #define ALT_PADDING_LEN(n1, n2) \
@@ -48,8 +55,8 @@ extern void alternative_instructions(void);
 
 #define OLDINSTR_2(oldinstr, n1, n2)                          \
     ".L%=_orig_s:\n\t" oldinstr "\n .L%=_orig_e:\n\t"         \
-    ".skip (-(("ALT_PADDING_LEN(n1, n2)") > 0) * "            \
-             "("ALT_PADDING_LEN(n1, n2)")), 0x90\n\t"         \
+    "mknops (-(("ALT_PADDING_LEN(n1, n2)") > 0) * "           \
+              "("ALT_PADDING_LEN(n1, n2)"))\n\t"              \
     ".L%=_orig_p:\n\t"
 
 #define ALTINSTR_ENTRY(feature, num)                                    \
-- 
2.1.4


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