linux-kernel - Re: [PATCH v4 14/18] static_call: Add static_cond

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Date:   Wed, 6 May 2020 12:24:55 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, pbonzini@...hat.com,
        mathieu.desnoyers@...icios.com
Subject: Re: [PATCH v4 14/18] static_call: Add static_cond_call()

On Fri, May 01, 2020 at 10:29:03PM +0200, Peter Zijlstra wrote:
> +++ b/arch/x86/include/asm/static_call.h
> @@ -30,4 +30,14 @@
>  	    ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
>  	    ".popsection					\n")
>  
> +#define ARCH_DEFINE_STATIC_CALL_RETTRAMP(name)				\
> +	asm(".pushsection .static_call.text, \"ax\"		\n"	\
> +	    ".align 4						\n"	\
> +	    ".globl " STATIC_CALL_TRAMP_STR(name) "		\n"	\
> +	    STATIC_CALL_TRAMP_STR(name) ":			\n"	\
> +	    "	ret; nop; nop; nop; nop;			\n"	\
> +	    ".type " STATIC_CALL_TRAMP_STR(name) ", @function	\n"	\
> +	    ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \
> +	    ".popsection					\n")
> +

The boilerplate in these two trampoline macros is identical except for
the actual instructions, maybe there could be a shared
__ARCH_DEFINE_STATIC_CALL_TRAMP(name, insns) macro which does most of
the dirty work.

>  #endif /* _ASM_STATIC_CALL_H */
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -4,19 +4,41 @@
>  #include <linux/bug.h>
>  #include <asm/text-patching.h>
>  
> -static void __static_call_transform(void *insn, u8 opcode, void *func)
> +enum insn_type {
> +	call = 0, /* site call */
> +	nop = 1,  /* site cond-call */
> +	jmp = 2,  /* tramp / site tail-call */
> +	ret = 3,  /* tramp / site cond-tail-call */
> +};

The lowercase enums threw me for a loop, I thought they were variables a
few times.  Starting a new enum trend? :-)

>  void arch_static_call_transform(void *site, void *tramp, void *func)
> @@ -24,10 +46,10 @@ void arch_static_call_transform(void *si
>  	mutex_lock(&text_mutex);
>  
>  	if (tramp)
> -		__static_call_transform(tramp, JMP32_INSN_OPCODE, func);
> +		__static_call_transform(tramp, jmp + !func, func);
>  
>  	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site)
> -		__static_call_transform(site, CALL_INSN_OPCODE, func);
> +		__static_call_transform(site, !func, func);

Clever enum math, but probably more robust to be ignorant of the values:

	if (tramp)
		__static_call_transform(tramp, func ? jmp : ret, func);

  	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site)
		__static_call_transform(site, func ? call : nop, func);


> +++ b/include/linux/static_call.h
> @@ -16,7 +16,9 @@
>   *
>   *   DECLARE_STATIC_CALL(name, func);
>   *   DEFINE_STATIC_CALL(name, func);
> + *   DEFINE_STATIC_COND_CALL(name, typename);
>   *   static_call(name)(args...);
> + *   static_cond_call(name)(args...)
>   *   static_call_update(name, func);

Missing semicolon, also an updated description/example would be useful.

On that note, what do you think about tweaking the naming from

  DEFINE_STATIC_COND_CALL(name, typename);
  static_cond_call(name)(args...);

to

  DEFINE_STATIC_CALL_NO_FUNC(name, typename);
  static_call_if_func(name)(args...);

?

Seems clearer to me.  They're still STATIC_CALLs, so it seems logical to
keep those two words together.  And NO_FUNC clarifies the initialized
value.

Similarly RETTRAMP could be ARCH_DEFINE_STATIC_CALL_NO_FUNC.

-- 
Josh