linux-kernel - Re: [PATCH 4/4] x86/entry/64: Bypass enter_from_user

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Date:	Mon, 9 Nov 2015 09:52:23 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 4/4] x86/entry/64: Bypass enter_from_user_mode on
 non-context-tracking boots


* Andy Lutomirski <luto@...nel.org> wrote:

> On CONFIG_CONTEXT_TRACKING kernels that have context tracking
> disabled at runtime (which includes most distro kernels), we still
> have the overhead of a call to enter_from_user_mode in interrupt and
> exception entries.
> 
> If jump labels are available, this uses the jump label
> infrastructure to skip the call.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/calling.h  | 15 +++++++++++++++
>  arch/x86/entry/entry_64.S |  8 ++------
>  2 files changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 3c71dd947c7b..271e30c585bc 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -1,3 +1,5 @@
> +#include <linux/jump_label.h>
> +
>  /*
>  
>   x86 function call convention, 64-bit:
> @@ -232,3 +234,16 @@ For 32-bit we have the following conventions - kernel is built with
>  
>  #endif /* CONFIG_X86_64 */
>  
> +/*
> + * This does 'call enter_from_user_mode' unless we can avoid it based on
> + * kernel config or using the static jump infrastructure.
> + */
> +.macro CALL_ENTER_FROM_USER_MODE
> +#ifdef CONFIG_CONTEXT_TRACKING
> +#ifdef HAVE_JUMP_LABEL
> +	STATIC_JUMP_IF_FALSE .Lafter_call_\@, context_tracking_enabled, def=0
> +#endif
> +	call enter_from_user_mode
> +.Lafter_call_\@:
> +#endif
> +.endm
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index f585df24ab3d..9b49b56efa29 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -517,9 +517,7 @@ END(irq_entries_start)
>  	 */
>  	TRACE_IRQS_OFF
>  
> -#ifdef CONFIG_CONTEXT_TRACKING
> -	call enter_from_user_mode
> -#endif
> +	CALL_ENTER_FROM_USER_MODE
>  
>  1:
>  	/*
> @@ -1058,9 +1056,7 @@ ENTRY(error_entry)
>  
>  .Lerror_entry_from_usermode_after_swapgs:
>  	TRACE_IRQS_OFF
> -#ifdef CONFIG_CONTEXT_TRACKING
> -	call enter_from_user_mode
> -#endif
> +	CALL_ENTER_FROM_USER_MODE
>  	ret

Yeah, so I only have a really minor syntactic nit abou tthis: it's OK to 
capitalize things when doing macros, but here I don't think capitalizing the 
called function name is good - I think the following would be more obvious:

	CALL_enter_from_user_mode

this makes it really, really obvious (to me!) that this macro is a shortcut for:

	call enter_from_user_mode

with some glue around it, and I could grep for 'enter_from_user_mode' immediately 
- while grepping for ENTER_FROM_USER_MODE would miss the called function.

Thanks,

	Ingo
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