linux-kernel - Re: [PATCH 2/2] x86/entry: Inline enter_from_user

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Date:	Thu, 9 Jun 2016 10:17:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2] x86/entry: Inline enter_from_user_mode

On Mon, Jun 6, 2016 at 9:01 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>
> On 04/06/2016 07:08, Andy Lutomirski wrote:
>> On May 30, 2016 5:30 AM, "Paolo Bonzini" <pbonzini@...hat.com> wrote:
>>>
>>> This matches what is already done for prepare_exit_to_usermode,
>>> and saves about 60 clock cycles (4% speedup) with the benchmark
>>> in the previous commit message.
>>>
>>> Cc: Andy Lutomirski <luto@...nel.org>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Cc: Rik van Riel <riel@...hat.com>
>>> Cc: H. Peter Anvin <hpa@...or.com>
>>> Cc: Ingo Molnar <mingo@...nel.org.com>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>>> ---
>>>  arch/x86/entry/common.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>>> index 946bc1a..582bbc8 100644
>>> --- a/arch/x86/entry/common.c
>>> +++ b/arch/x86/entry/common.c
>>> @@ -40,7 +40,7 @@ static struct thread_info *pt_regs_to_thread_info(struct pt_regs *regs)
>>>
>>>  #ifdef CONFIG_CONTEXT_TRACKING
>>>  /* Called on entry from user mode with IRQs off. */
>>> -__visible void enter_from_user_mode(void)
>>> +__visible inline void enter_from_user_mode(void)
>>>  {
>>>         CT_WARN_ON(ct_state() != CONTEXT_USER);
>>>         __user_exit();
>>
>> I wonder if an extern inline *declaration* is needed as well in this C
>> file.  At least C99 suggests it is.  Maybe __visible is sufficient to
>> force an external definition to be emitted.
>
> An extern inline declaration is not needed because the kernel uses
> -std=gnu89 (or, if you prefer, because prepare_exit_to_usermode didn't
> have one :)).
>
> It's awesomely perverted:
>
>   __attribute__((externally_visible)) inline void f(void) {}
>
>   inline void g(void) {}
>
>   extern inline void h(void);
>   extern inline void h(void) {}
>
>   inline void i(void);
>   inline void i(void) {}
>
>   extern inline void j(void);
>   inline void j(void) {}
>
> This patch (and the preexisting prepare_exit_to_usermode code) are
> equivalent to "f".
>
> Compile the above file with "--std=gnu89" or "--std=gnu99
> -fgnu89-inline" and f/g/i/j are emitted.
>
> Compile it with "--std=gnu99 -fno-gnu89-inline" and h/j is emitted.
>
> Yes, the standard is _almost exactly_ the opposite of the preexisting
> GCC implementation.  The only case which achieves the same effect is
> when declarations are "extern inline" and definitions must always be
> "inline".  Or of course just use "static inline".  At least it's
> decently documented in the GCC info documentation.

OK.

In any event, if this ever messes up, it'll fail to build and we'll notice.

--Andy