linux-kernel - Re: [PATCH v5 4/7] arm64: introduce aarch64_insn_gen_{nop|branch

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Message-ID: <20131030004840.GC25346@mudshark.cambridge.arm.com>
Date:	Wed, 30 Oct 2013 00:48:40 +0000
From:	Will Deacon <will.deacon@....com>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Jiang Liu <jiang.liu@...wei.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/7] arm64: introduce
 aarch64_insn_gen_{nop|branch_imm}() helper functions

On Fri, Oct 18, 2013 at 04:19:58PM +0100, Jiang Liu wrote:
> From: Jiang Liu <jiang.liu@...wei.com>
> 
> Introduce aarch64_insn_gen_{nop|branch_imm}() helper functions, which
> will be used to implement jump label on ARM64.
> 
> Signed-off-by: Jiang Liu <jiang.liu@...wei.com>
> Cc: Jiang Liu <liuj97@...il.com>
> ---
>  arch/arm64/include/asm/insn.h | 13 +++++++++++++
>  arch/arm64/kernel/insn.c      | 28 ++++++++++++++++++++++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
> index 8f94e48..ee4a60d 100644
> --- a/arch/arm64/include/asm/insn.h
> +++ b/arch/arm64/include/asm/insn.h
> @@ -96,9 +96,22 @@ static __always_inline void aarch64_insn_write(void *addr, u32 insn)
>  	*(u32 *)addr = cpu_to_le32(insn);
>  }
>  
> +static __always_inline u32 aarch64_insn_gen_hint(enum aarch64_insn_hint_op op)
> +{
> +	return aarch64_insn_get_hint_value() | op;

What's the point in that helper function? Does it just return an immediate
value (and are there are other users of it?).

> +}
> +
> +static __always_inline u32 aarch64_insn_gen_nop(void)
> +{
> +	return aarch64_insn_gen_hint(AARCH64_INSN_HINT_NOP);
> +}

Either use plain old `inline' or write these as preprocessor macros.

> +
>  enum aarch64_insn_encoding_class aarch64_get_insn_class(u32 insn);
>  u32 aarch64_insn_encode_immediate(enum aarch64_insn_imm_type type,
>  				  u32 insn, u64 imm);
> +u32 aarch64_insn_gen_branch_imm(unsigned long pc, unsigned long addr,
> +				bool link);
> +
>  bool aarch64_insn_hotpatch_safe(u32 old_insn, u32 new_insn);
>  
>  int aarch64_insn_patch_text_nosync(void *addr, u32 insn);
> diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
> index 886821f..f7498cc 100644
> --- a/arch/arm64/kernel/insn.c
> +++ b/arch/arm64/kernel/insn.c
> @@ -14,6 +14,7 @@
>   * 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/bitops.h>
>  #include <linux/compiler.h>
>  #include <linux/kernel.h>
>  #include <linux/smp.h>
> @@ -233,3 +234,30 @@ u32 aarch64_insn_encode_immediate(enum aarch64_insn_imm_type type,
>  
>  	return insn;
>  }
> +
> +u32 aarch64_insn_gen_branch_imm(unsigned long pc, unsigned long addr, bool link)
> +{
> +	u32 insn;
> +	long offset;
> +
> +	/*
> +	 * PC: A 64-bit Program Counter holding the address of the current
> +	 * instruction. A64 instructions may be word-aligned.
> +	 */
> +	BUG_ON((pc & 0x3) || (addr & 0x3));
> +
> +	/*
> +	 * B/BL support [-128M, 128M) offset
> +	 * ARM64 virtual address arrangement garantees all kernel and module

Typo: `guarantees'.

> +	 * texts are within +/-128M.
> +	 */
> +	offset = ((long)addr - (long)pc) >> 2;
> +	BUG_ON(abs(offset) > BIT(25) || offset == BIT(25));

I really struggle to follow this range checking. Why don't you just compare
the absolute difference between addr and pc with SZ_128M?

Will
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