diff options
| author | Peter Zijlstra <peterz@infradead.org> | 2021-03-12 12:32:54 +0100 |
|---|---|---|
| committer | Borislav Petkov <bp@suse.de> | 2021-03-15 16:24:59 +0100 |
| commit | a89dfde3dc3c2dbf56910af75e2d8b11ec5308f6 (patch) | |
| tree | 1f5d63a4ea45af7d513ce044362993174ad52694 /arch/x86/include/asm/special_insns.h | |
| parent | 59eca2fa1934de42d8aa44d3bef655c92ea69703 (diff) | |
x86: Remove dynamic NOP selection
This ensures that a NOP is a NOP and not a random other instruction that
is also a NOP. It allows simplification of dynamic code patching that
wants to verify existing code before writing new instructions (ftrace,
jump_label, static_call, etc..).
Differentiating on NOPs is not a feature.
This pessimises 32bit (DONTCARE) and 32bit on 64bit CPUs (CARELESS).
32bit is not a performance target.
Everything x86_64 since AMD K10 (2007) and Intel IvyBridge (2012) is
fine with using NOPL (as opposed to prefix NOP). And per FEATURE_NOPL
being required for x86_64, all x86_64 CPUs can use NOPL. So stop
caring about NOPs, simplify things and get on with life.
[ The problem seems to be that some uarchs can only decode NOPL on a
single front-end port while others have severe decode penalties for
excessive prefixes. All modern uarchs can handle both, except Atom,
which has prefix penalties. ]
[ Also, much doubt you can actually measure any of this on normal
workloads. ]
After this, FEATURE_NOPL is unused except for required-features for
x86_64. FEATURE_K8 is only used for PTI.
[ bp: Kernel build measurements showed ~0.3s slowdown on Sandybridge
which is hardly a slowdown. Get rid of X86_FEATURE_K7, while at it. ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Alexei Starovoitov <alexei.starovoitov@gmail.com> # bpf
Acked-by: Linus Torvalds <torvalds@linuxfoundation.org>
Link: https://lkml.kernel.org/r/20210312115749.065275711@infradead.org
Diffstat (limited to 'arch/x86/include/asm/special_insns.h')
| -rw-r--r-- | arch/x86/include/asm/special_insns.h | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h index 1d3cbaef4bb7..2acd6cb62328 100644 --- a/arch/x86/include/asm/special_insns.h +++ b/arch/x86/include/asm/special_insns.h @@ -214,7 +214,7 @@ static inline void clflush(volatile void *__p) static inline void clflushopt(volatile void *__p) { - alternative_io(".byte " __stringify(NOP_DS_PREFIX) "; clflush %P0", + alternative_io(".byte 0x3e; clflush %P0", ".byte 0x66; clflush %P0", X86_FEATURE_CLFLUSHOPT, "+m" (*(volatile char __force *)__p)); @@ -225,7 +225,7 @@ static inline void clwb(volatile void *__p) volatile struct { char x[64]; } *p = __p; asm volatile(ALTERNATIVE_2( - ".byte " __stringify(NOP_DS_PREFIX) "; clflush (%[pax])", + ".byte 0x3e; clflush (%[pax])", ".byte 0x66; clflush (%[pax])", /* clflushopt (%%rax) */ X86_FEATURE_CLFLUSHOPT, ".byte 0x66, 0x0f, 0xae, 0x30", /* clwb (%%rax) */ |