From a89dfde3dc3c2dbf56910af75e2d8b11ec5308f6 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 12 Mar 2021 12:32:54 +0100 Subject: 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) Signed-off-by: Borislav Petkov Acked-by: Alexei Starovoitov # bpf Acked-by: Linus Torvalds Link: https://lkml.kernel.org/r/20210312115749.065275711@infradead.org --- arch/x86/kernel/kprobes/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'arch/x86/kernel/kprobes/core.c') diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c index df776cdca327..6356834928b9 100644 --- a/arch/x86/kernel/kprobes/core.c +++ b/arch/x86/kernel/kprobes/core.c @@ -229,7 +229,7 @@ __recover_probed_insn(kprobe_opcode_t *buf, unsigned long addr) return 0UL; if (faddr) - memcpy(buf, ideal_nops[NOP_ATOMIC5], 5); + memcpy(buf, x86_nops[5], 5); else buf[0] = kp->opcode; return (unsigned long)buf; -- cgit v1.2.3