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Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
To:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <eb15f2dd-92f3-423d-e22f-3d1528fb7b6e@rasmusvillemoes.dk>
Date:   Fri, 30 Nov 2018 23:16:34 +0100
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On 29/11/2018 20.22, Josh Poimboeuf wrote:
> On Thu, Nov 29, 2018 at 02:16:48PM -0500, Steven Rostedt wrote:
>>> and honestly, the way "static_call()" works now, can you guarantee
>>> that the call-site doesn't end up doing that, and calling the
>>> trampoline function for two different static calls from one indirect
>>> call?
>>>
>>> See what I'm talking about? Saying "callers are wrapped in macros"
>>> doesn't actually protect you from the compiler doing things like that.
>>>
>>> In contrast, if the call was wrapped in an inline asm, we'd *know* the
>>> compiler couldn't turn a "call wrapper(%rip)" into anything else.
>>
>> But then we need to implement all numbers of parameters.
> 
> I actually have an old unfinished patch which (ab)used C macros to
> detect the number of parameters and then setup the asm constraints
> accordingly.  At the time, the goal was to optimize the BUG code.
> 
> I had wanted to avoid this kind of approach for static calls, because
> "ugh", but now it's starting to look much more appealing.
> 
> Behold:
> 
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index aa6b2023d8f8..d63e9240da77 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -32,10 +32,59 @@
>  
>  #ifdef CONFIG_DEBUG_BUGVERBOSE
>  
> -#define _BUG_FLAGS(ins, flags)						\
> +#define __BUG_ARGS_0(ins, ...) \
> +({\
> +	asm volatile("1:\t" ins "\n"); \
> +})
> +#define __BUG_ARGS_1(ins, ...) \
> +({\
> +	asm volatile("1:\t" ins "\n" \
> +		     : : "D" (ARG1(__VA_ARGS__))); \
> +})
> +#define __BUG_ARGS_2(ins, ...) \
> +({\
> +	asm volatile("1:\t" ins "\n" \
> +		     : : "D" (ARG1(__VA_ARGS__)), \
> +			 "S" (ARG2(__VA_ARGS__))); \
> +})
> +#define __BUG_ARGS_3(ins, ...) \
> +({\
> +	asm volatile("1:\t" ins "\n" \
> +		     : : "D" (ARG1(__VA_ARGS__)), \
> +			 "S" (ARG2(__VA_ARGS__)), \
> +			 "d" (ARG3(__VA_ARGS__))); \
> +})

wouldn't you need to tie all these to (unused) outputs as well as adding
the remaining caller-saved registers to the clobber list? Maybe not for
the WARN machinery(?), but at least for stuff that should look like a
normal call to gcc? Then there's %rax which is either a clobber or an
output, and if there's not to be a separate static_call_void(), one
would need to do some __builtin_choose_expr(__same_type(void, f(...)), ...).

Rasmus