Re: [PATCH v3 6/7] arm64, jump label: optimize jump labelimplementation

[Will Deacon]

On Wed, Oct 16, 2013 at 04:18:11AM +0100, Jiang Liu wrote:
> From: Jiang Liu <jiang.liu@xxxxxxxxxx>
> 
> Optimize jump label implementation for ARM64 by dynamically patching
> kernel text.
> 
> Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxx>
> Cc: Jiang Liu <liuj97@xxxxxxxxx>
> ---
>  arch/arm64/Kconfig                  |  1 +
>  arch/arm64/include/asm/jump_label.h | 52 ++++++++++++++++++++++++++++++++
>  arch/arm64/kernel/Makefile          |  1 +
>  arch/arm64/kernel/jump_label.c      | 60 +++++++++++++++++++++++++++++++++++++
>  4 files changed, 114 insertions(+)
>  create mode 100644 arch/arm64/include/asm/jump_label.h
>  create mode 100644 arch/arm64/kernel/jump_label.c
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index c044548..da388e4 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -17,6 +17,7 @@ config ARM64
>  	select GENERIC_SMP_IDLE_THREAD
>  	select GENERIC_TIME_VSYSCALL
>  	select HARDIRQS_SW_RESEND
> +	select HAVE_ARCH_JUMP_LABEL
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_DEBUG_BUGVERBOSE
>  	select HAVE_DEBUG_KMEMLEAK
> diff --git a/arch/arm64/include/asm/jump_label.h b/arch/arm64/include/asm/jump_label.h
> new file mode 100644
> index 0000000..d268fab
> --- /dev/null
> +++ b/arch/arm64/include/asm/jump_label.h
> @@ -0,0 +1,52 @@
> +/*
> + * Copyright (C) 2013 Huawei Ltd.
> + * Author: Jiang Liu <jiang.liu@xxxxxxxxxx>
> + *
> + * Based on arch/arm/include/asm/jump_label.h
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +#ifndef _ASM_ARM64_JUMP_LABEL_H
> +#define _ASM_ARM64_JUMP_LABEL_H
> +#include <linux/types.h>
> +
> +#ifdef __KERNEL__
> +
> +#define JUMP_LABEL_NOP_SIZE 4
> +
> +static __always_inline bool arch_static_branch(struct static_key *key)
> +{
> +	asm goto("1:\n\t"
> +		 "nop\n\t"
> +		 ".pushsection __jump_table,  \"aw\"\n\t"
> +		 ".align 3\n\t"
> +		 ".quad 1b, %l[l_yes], %c0\n\t"
> +		 ".popsection\n\t"
> +		 :  :  "i"(key) :  : l_yes);
> +
> +	return false;
> +l_yes:
> +	return true;
> +}
> +
> +#endif /* __KERNEL__ */
> +
> +typedef u64 jump_label_t;
> +
> +struct jump_entry {
> +	jump_label_t code;
> +	jump_label_t target;
> +	jump_label_t key;
> +};
> +
> +#endif	/* _ASM_ARM64_JUMP_LABEL_H */
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index 9af6cb3..b7db65e 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -18,6 +18,7 @@ arm64-obj-$(CONFIG_SMP)			+= smp.o smp_spin_table.o smp_psci.o
>  arm64-obj-$(CONFIG_HW_PERF_EVENTS)	+= perf_event.o
>  arm64-obj-$(CONFIG_HAVE_HW_BREAKPOINT)+= hw_breakpoint.o
>  arm64-obj-$(CONFIG_EARLY_PRINTK)	+= early_printk.o
> +arm64-obj-$(CONFIG_JUMP_LABEL)		+= jump_label.o
>  
>  obj-y					+= $(arm64-obj-y) vdso/
>  obj-m					+= $(arm64-obj-m)
> diff --git a/arch/arm64/kernel/jump_label.c b/arch/arm64/kernel/jump_label.c
> new file mode 100644
> index 0000000..74cbc73
> --- /dev/null
> +++ b/arch/arm64/kernel/jump_label.c
> @@ -0,0 +1,60 @@
> +/*
> + * Copyright (C) 2013 Huawei Ltd.
> + * Author: Jiang Liu <jiang.liu@xxxxxxxxxx>
> + *
> + * Based on arch/arm/kernel/jump_label.c
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +#include <linux/kernel.h>
> +#include <linux/jump_label.h>
> +#include <asm/jump_label.h>
> +#include <asm/insn.h>
> +
> +#ifdef HAVE_JUMP_LABEL
> +
> +static void __arch_jump_label_transform(struct jump_entry *entry,
> +					enum jump_label_type type,
> +					bool is_static)
> +{
> +	void *addr = (void *)entry->code;
> +	u32 insn;
> +
> +	if (type == JUMP_LABEL_ENABLE) {
> +		/* no way out if instruction offset is out of range(+/-128M) */
> +		insn = aarch64_insn_gen_branch_imm(entry->code,
> +						   entry->target, 0);
> +		BUG_ON(!insn);

In this case, rather than BUG_ON, it would be better (somehow) to abort
optimisation and instead patch in a branch to the actual condition check.

> +	} else {
> +		insn = aarch64_insn_gen_nop();

You could make the code more concise by limiting your patching ability to
branch immediates. Then a nop is simply a branch to the next instruction (I
doubt any modern CPUs will choke on this, whereas the architecture requires
a NOP to take time).

Will
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