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SubjectRe: [PATCH v3 6/7] arm64, jump label: optimize jump label implementation
On Wed, Oct 16, 2013 at 04:18:11AM +0100, Jiang Liu wrote:
> From: Jiang Liu <jiang.liu@huawei.com>
>
> Optimize jump label implementation for ARM64 by dynamically patching
> kernel text.
>
> Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
> Cc: Jiang Liu <liuj97@gmail.com>
> ---
> 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@huawei.com>
> + *
> + * 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@huawei.com>
> + *
> + * 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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