linux-kernel - Re: [PATCH 3/3] rseq/selftests: Add support for arm64

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Date:   Mon, 25 Jun 2018 14:10:10 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        peter maydell <peter.maydell@...aro.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 3/3] rseq/selftests: Add support for arm64

----- On Jun 25, 2018, at 1:54 PM, Will Deacon will.deacon@....com wrote:

[...]

> +#define __RSEQ_ASM_DEFINE_TABLE(label, version, flags, start_ip,		\
> +				post_commit_offset, abort_ip)			\
> +	"	.pushsection	__rseq_table, \"aw\"\n"				\
> +	"	.balign	32\n"							\
> +	__rseq_str(label) ":\n"							\
> +	"	.long	" __rseq_str(version) ", " __rseq_str(flags) "\n"	\
> +	"	.quad	" __rseq_str(start_ip) ", "				\
> +			  __rseq_str(post_commit_offset) ", "			\
> +			  __rseq_str(abort_ip) "\n"				\
> +	"	.popsection\n"
> +
> +#define RSEQ_ASM_DEFINE_TABLE(label, start_ip, post_commit_ip, abort_ip)	\
> +	__RSEQ_ASM_DEFINE_TABLE(label, 0x0, 0x0, start_ip,			\
> +				(post_commit_ip - start_ip), abort_ip)
> +
> +#define RSEQ_ASM_STORE_RSEQ_CS(label, cs_label, rseq_cs)			\
> +	RSEQ_INJECT_ASM(1)							\
> +	"	adrp	" RSEQ_ASM_TMP_REG ", " __rseq_str(cs_label) "\n"	\
> +	"	add	" RSEQ_ASM_TMP_REG ", " RSEQ_ASM_TMP_REG		\
> +			", :lo12:" __rseq_str(cs_label) "\n"			\
> +	"	str	" RSEQ_ASM_TMP_REG ", %[" __rseq_str(rseq_cs) "]\n"	\
> +	__rseq_str(label) ":\n"
> +
> +#define RSEQ_ASM_DEFINE_ABORT(label, abort_label)				\
> +	"	.pushsection	__rseq_failure, \"ax\"\n"			\
> +	"	.long 	"	__rseq_str(RSEQ_SIG) "\n"			\
> +	__rseq_str(label) ":\n"							\
> +	"	b	%l[" __rseq_str(abort_label) "]\n"			\
> +	"	.popsection\n"

Thanks Will for porting rseq to arm64 !

I notice you are using the instructions

  adrp
  add
  str

to implement RSEQ_ASM_STORE_RSEQ_CS(). Did you compare
performance-wise with an approach using a literal pool
near the instruction pointer like I did on arm32 ?

With that approach, this ends up being simply

  adr
  str

which provides significantly better performance on my test
platform over loading a pointer targeting a separate data
section.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com