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Message-ID: <YE+BFIil8wj+q8Qi@hirez.programming.kicks-ass.net>
Date: Mon, 15 Mar 2021 16:45:24 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
Subject: [RFC][PATCH] x86/alternatives: Optimize optimize_nops()
Currently optimize_nops() scans to see if the alternative starts with
NOPs. However, the emit pattern is:
141: \oldinstr
142: .skip (len-(142b-141b)), 0x90
That is, when oldinstr is short, we pad the tail with NOPs. This case
never gets optimized.
Rewrite optimize_nops() to replace any string of NOPs inside the
alternative to larger NOPs. Also run it irrespective of patching,
replacing NOPs in both the original and replaced code.
A direct consequence is that padlen becomes superfluous, so remove it.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/x86/include/asm/alternative.h | 14 ++---
arch/x86/kernel/alternative.c | 63 ++++++++++++++++----------
tools/objtool/arch/x86/include/arch/special.h | 2
3 files changed, 46 insertions(+), 33 deletions(-)
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -65,7 +65,6 @@ struct alt_instr {
u16 cpuid; /* cpuid bit set for replacement */
u8 instrlen; /* length of original instruction */
u8 replacementlen; /* length of new instruction */
- u8 padlen; /* length of build-time padding */
} __packed;
/*
@@ -104,7 +103,6 @@ static inline int alternatives_text_rese
#define alt_end_marker "663"
#define alt_slen "662b-661b"
-#define alt_pad_len alt_end_marker"b-662b"
#define alt_total_slen alt_end_marker"b-661b"
#define alt_rlen(num) e_replacement(num)"f-"b_replacement(num)"f"
@@ -151,8 +149,7 @@ static inline int alternatives_text_rese
" .long " b_replacement(num)"f - .\n" /* new instruction */ \
" .word " __stringify(feature) "\n" /* feature bit */ \
" .byte " alt_total_slen "\n" /* source len */ \
- " .byte " alt_rlen(num) "\n" /* replacement len */ \
- " .byte " alt_pad_len "\n" /* pad len */
+ " .byte " alt_rlen(num) "\n" /* replacement len */
#define ALTINSTR_REPLACEMENT(newinstr, num) /* replacement */ \
"# ALT: replacement " #num "\n" \
@@ -315,13 +312,12 @@ static inline int alternatives_text_rese
* enough information for the alternatives patching code to patch an
* instruction. See apply_alternatives().
*/
-.macro altinstruction_entry orig alt feature orig_len alt_len pad_len
+.macro altinstruction_entry orig alt feature orig_len alt_len
.long \orig - .
.long \alt - .
.word \feature
.byte \orig_len
.byte \alt_len
- .byte \pad_len
.endm
/*
@@ -338,7 +334,7 @@ static inline int alternatives_text_rese
142:
.pushsection .altinstructions,"a"
- altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f,142b-141b
+ altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f
.popsection
.pushsection .altinstr_replacement,"ax"
@@ -375,8 +371,8 @@ static inline int alternatives_text_rese
142:
.pushsection .altinstructions,"a"
- altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f,142b-141b
- altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f,142b-141b
+ altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f
+ altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f
.popsection
.pushsection .altinstr_replacement,"ax"
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -345,19 +345,39 @@ recompute_jump(struct alt_instr *a, u8 *
static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr)
{
unsigned long flags;
- int i;
+ int nops = 0, i = 0;
+ struct insn insn;
+ u8 *nop = NULL;
+
+ do {
+ kernel_insn_init(&insn, &instr[i], MAX_INSN_SIZE);
+ insn_get_length(&insn);
+
+ if (insn.length == 1 && insn.opcode.bytes[0] == 0x90) {
+ if (!nop) {
+ nop = &instr[i];
+ nops = 1;
+ } else {
+ nops++;
+ }
+ }
+ i += insn.length;
- for (i = 0; i < a->padlen; i++) {
- if (instr[i] != 0x90)
- return;
- }
+ if ((insn.length != 1 || i == a->instrlen) && nop) {
+
+ local_irq_save(flags);
+ add_nops(nop, nops);
+ local_irq_restore(flags);
+
+ DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
+ instr, (int)(unsigned long)(nop-instr), nops);
+
+ nop = NULL;
+ }
- local_irq_save(flags);
- add_nops(instr + (a->instrlen - a->padlen), a->padlen);
- local_irq_restore(flags);
+ } while (i < a->instrlen);
- DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
- instr, a->instrlen - a->padlen, a->padlen);
+ WARN_ON_ONCE(nop);
}
/*
@@ -403,19 +423,15 @@ void __init_or_module noinline apply_alt
* - feature not present but ALTINSTR_FLAG_INV is set to mean,
* patch if feature is *NOT* present.
*/
- if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV)) {
- if (a->padlen > 1)
- optimize_nops(a, instr);
+ if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV))
+ goto next;
- continue;
- }
-
- DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d",
+ DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d)",
(a->cpuid & ALTINSTR_FLAG_INV) ? "!" : "",
feature >> 5,
feature & 0x1f,
instr, instr, a->instrlen,
- replacement, a->replacementlen, a->padlen);
+ replacement, a->replacementlen);
DUMP_BYTES(instr, a->instrlen, "%px: old_insn: ", instr);
DUMP_BYTES(replacement, a->replacementlen, "%px: rpl_insn: ", replacement);
@@ -439,14 +455,15 @@ void __init_or_module noinline apply_alt
if (a->replacementlen && is_jmp(replacement[0]))
recompute_jump(a, instr, replacement, insn_buff);
- if (a->instrlen > a->replacementlen) {
- add_nops(insn_buff + a->replacementlen,
- a->instrlen - a->replacementlen);
- insn_buff_sz += a->instrlen - a->replacementlen;
- }
+ for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
+ insn_buff[insn_buff_sz] = 0x90;
+
DUMP_BYTES(insn_buff, insn_buff_sz, "%px: final_insn: ", instr);
text_poke_early(instr, insn_buff, insn_buff_sz);
+
+next:
+ optimize_nops(a, instr);
}
}
--- a/tools/objtool/arch/x86/include/arch/special.h
+++ b/tools/objtool/arch/x86/include/arch/special.h
@@ -10,7 +10,7 @@
#define JUMP_ORIG_OFFSET 0
#define JUMP_NEW_OFFSET 4
-#define ALT_ENTRY_SIZE 13
+#define ALT_ENTRY_SIZE 12
#define ALT_ORIG_OFFSET 0
#define ALT_NEW_OFFSET 4
#define ALT_FEATURE_OFFSET 8
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