linux-kernel - Re: [PATCH V2 3/3] arm64/kprobe: Optimize the performance of patching single-step slot

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Message-ID: <20220922133850.GC12095@willie-the-truck>
Date:   Thu, 22 Sep 2022 14:38:51 +0100
From:   Will Deacon <will@...nel.org>
To:     Liao Chang <liaochang1@...wei.com>
Cc:     catalin.marinas@....com, guoren@...nel.org,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, rostedt@...dmis.org, mhiramat@...nel.org,
        maz@...nel.org, alexandru.elisei@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-csky@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH V2 3/3] arm64/kprobe: Optimize the performance of
 patching single-step slot

On Tue, Sep 13, 2022 at 11:34:54AM +0800, Liao Chang wrote:
> Single-step slot would not be used until kprobe is enabled, that means
> no race condition occurs on it under SMP, hence it is safe to pacth ss
> slot without stopping machine.
> 
> Signed-off-by: Liao Chang <liaochang1@...wei.com>
> ---
>  arch/arm64/kernel/probes/kprobes.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index d1d182320245..5902e33fd3b6 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -44,11 +44,10 @@ post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
>  static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
>  {
>  	kprobe_opcode_t *addr = p->ainsn.api.insn;
> -	void *addrs[] = {addr, addr + 1};
> -	u32 insns[] = {p->opcode, BRK64_OPCODE_KPROBES_SS};
>  
>  	/* prepare insn slot */
> -	aarch64_insn_patch_text(addrs, insns, 2);
> +	aarch64_insn_write(addr, p->opcode);
> +	aarch64_insn_write(addr + 1, BRK64_OPCODE_KPROBES_SS);
>  
>  	flush_icache_range((uintptr_t)addr, (uintptr_t)(addr + MAX_INSN_SIZE));

Hmm, so it looks like prior to your change we were doing the cache
maintebnance twice: once in aarch64_insn_patch_text() from stop machine
context and then again in the flush_icache_range() call above.

I suppose the cleanest thing would be to drop the flush_icache_range()
call and then use aarch64_insn_patch_text_nosync() instead of
aarch64_insn_write() in your change.

Will