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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Newer versions of binutils are capable of emitting an exact number bytes worth
of optimised nops, which are P6 nops.  Use this in preference to .skip when
available.

Check at boot time whether the toolchain nops are the correct for the running
hardware, andskip optimising nops entirely when possible.

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>
---
 xen/arch/x86/Rules.mk                 |  4 ++++
 xen/arch/x86/alternative.c            | 20 +++++++++++++++++++-
 xen/include/asm-x86/alternative-asm.h | 12 +++++++++++-
 xen/include/asm-x86/alternative.h     | 11 +++++++++--
 4 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index ac585a3..c84ed20 100644
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -29,6 +29,10 @@ $(call as-option-add,CFLAGS,CC,"invpcid 
(%rax)$$(comma)%rax",-DHAVE_AS_INVPCID)
 $(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: .nops (.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/arch/x86/alternative.c b/xen/arch/x86/alternative.c
index 0ef7a8b..2c844d6 100644
--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -84,6 +84,19 @@ static const unsigned char * const p6_nops[ASM_NOP_MAX+1] 
init_or_livepatch_cons
 
 static const unsigned char * const *ideal_nops init_or_livepatch_data = 
p6_nops;
 
+#ifdef HAVE_AS_NOP_DIRECTIVE
+
+/* Nops in .init.rodata to compare against the runtime ideal nops. */
+asm ( ".pushsection .init.rodata, \"a\", @progbits\n\t"
+      "toolchain_nops: .nops " __stringify(ASM_NOP_MAX) "\n\t"
+      ".popsection\n\t");
+extern char toolchain_nops[ASM_NOP_MAX];
+static bool __read_mostly toolchain_nops_are_ideal;
+
+#else
+# define toolchain_nops_are_ideal false
+#endif
+
 static void __init arch_init_ideal_nops(void)
 {
     switch ( boot_cpu_data.x86_vendor )
@@ -112,6 +125,11 @@ static void __init arch_init_ideal_nops(void)
             ideal_nops = k8_nops;
         break;
     }
+
+#ifdef HAVE_AS_NOP_DIRECTIVE
+    if ( memcmp(ideal_nops[ASM_NOP_MAX], toolchain_nops, ASM_NOP_MAX) == 0 )
+        toolchain_nops_are_ideal = true;
+#endif
 }
 
 /* Use this to add nops to a buffer, then text_poke the whole buffer. */
@@ -209,7 +227,7 @@ void init_or_livepatch apply_alternatives(struct alt_instr 
*start,
             base->priv = 1;
 
             /* Nothing useful to do? */
-            if ( a->pad_len <= 1 )
+            if ( toolchain_nops_are_ideal || a->pad_len <= 1 )
                 continue;
 
             add_nops(buf, a->pad_len);
diff --git a/xen/include/asm-x86/alternative-asm.h 
b/xen/include/asm-x86/alternative-asm.h
index 0b61516..0d6fb4b 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__
 
 /*
@@ -19,6 +21,14 @@
     .byte 0 /* priv */
 .endm
 
+.macro mknops nr_bytes
+#ifdef HAVE_AS_NOP_DIRECTIVE
+    .nops \nr_bytes, ASM_NOP_MAX
+#else
+    .skip \nr_bytes, 0x90
+#endif
+.endm
+
 /* GAS's idea of true is -1, while Clang's idea is 1. */
 #ifdef HAVE_AS_NEGATIVE_TRUE
 # define as_true(x) (-(x))
@@ -29,7 +39,7 @@
 #define decl_orig(insn, padding)                  \
  .L\@_orig_s: insn; .L\@_orig_e:                  \
  .L\@_diff = padding;                             \
- .skip as_true(.L\@_diff > 0) * .L\@_diff, 0x90;  \
+ mknops (as_true(.L\@_diff > 0) * .L\@_diff);     \
  .L\@_orig_p:
 
 #define orig_len               (.L\@_orig_e       -     .L\@_orig_s)
diff --git a/xen/include/asm-x86/alternative.h 
b/xen/include/asm-x86/alternative.h
index 619472e..84b4854 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 add_nops(void *insns, unsigned int len);
 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_NOP_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)"
@@ -46,7 +53,7 @@ extern void alternative_instructions(void);
 #define OLDINSTR(oldinstr, padding)                              \
     ".LXEN%=_orig_s:\n\t" oldinstr "\n .LXEN%=_orig_e:\n\t"      \
     ".LXEN%=_diff = " padding "\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 OLDINSTR_1(oldinstr, n1)                                 \
-- 
2.1.4


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