linux-kernel - Re: [PATCH 3/3] x86_64,entry: Remove the syscall exit audit and schedule optimizations

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Date:	Fri, 9 Jan 2015 16:53:35 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>, kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH 3/3] x86_64,entry: Remove the syscall exit audit and
 schedule optimizations

On Fri, Nov 07, 2014 at 03:58:19PM -0800, Andy Lutomirski wrote:
> We used to optimize rescheduling and audit on syscall exit.  Now that
> the full slow path is reasonably fast, remove these optimizations.
> 
> This adds something like 10ns to the previously optimized paths on my
> computer, presumably due mostly to SAVE_REST / RESTORE_REST.
> 
> I think that we should eventually replace both the syscall and
> non-paranoid interrupt exit slow paths with a pair of C functions
> along the lines of the syscall entry hooks.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/kernel/entry_64.S | 52 +++++-----------------------------------------
>  1 file changed, 5 insertions(+), 47 deletions(-)
> 
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index a5afdf0f7fa4..222dc5c45ac3 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -427,15 +427,12 @@ system_call_fastpath:
>   * Has incomplete stack frame and undefined top of stack.
>   */
>  ret_from_sys_call:
> -	movl $_TIF_ALLWORK_MASK,%edi
> -	/* edi:	flagmask */
> -sysret_check:
> +	testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
> +	jnz int_ret_from_sys_call_fixup	/* Go the the slow path */
> +
>  	LOCKDEP_SYS_EXIT
>  	DISABLE_INTERRUPTS(CLBR_NONE)
>  	TRACE_IRQS_OFF
> -	movl TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET),%edx
> -	andl %edi,%edx
> -	jnz  sysret_careful
>  	CFI_REMEMBER_STATE
>  	/*
>  	 * sysretq will re-enable interrupts:
> @@ -449,49 +446,10 @@ sysret_check:
>  	USERGS_SYSRET64
>  
>  	CFI_RESTORE_STATE
> -	/* Handle reschedules */
> -	/* edx:	work, edi: workmask */
> -sysret_careful:
> -	bt $TIF_NEED_RESCHED,%edx
> -	jnc sysret_signal
> -	TRACE_IRQS_ON
> -	ENABLE_INTERRUPTS(CLBR_NONE)
> -	pushq_cfi %rdi
> -	SCHEDULE_USER
> -	popq_cfi %rdi
> -	jmp sysret_check
>  
> -	/* Handle a signal */
> -sysret_signal:
> -	TRACE_IRQS_ON
> -	ENABLE_INTERRUPTS(CLBR_NONE)
> -#ifdef CONFIG_AUDITSYSCALL
> -	bt $TIF_SYSCALL_AUDIT,%edx
> -	jc sysret_audit
> -#endif
> -	/*
> -	 * We have a signal, or exit tracing or single-step.
> -	 * These all wind up with the iret return path anyway,
> -	 * so just join that path right now.
> -	 */
> +int_ret_from_sys_call_fixup:
>  	FIXUP_TOP_OF_STACK %r11, -ARGOFFSET
> -	jmp int_check_syscall_exit_work
> -
> -#ifdef CONFIG_AUDITSYSCALL
> -	/*
> -	 * Return fast path for syscall audit.  Call __audit_syscall_exit()
> -	 * directly and then jump back to the fast path with TIF_SYSCALL_AUDIT
> -	 * masked off.
> -	 */
> -sysret_audit:
> -	movq RAX-ARGOFFSET(%rsp),%rsi	/* second arg, syscall return value */
> -	cmpq $-MAX_ERRNO,%rsi	/* is it < -MAX_ERRNO? */
> -	setbe %al		/* 1 if so, 0 if not */
> -	movzbl %al,%edi		/* zero-extend that into %edi */
> -	call __audit_syscall_exit
> -	movl $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT),%edi
> -	jmp sysret_check
> -#endif	/* CONFIG_AUDITSYSCALL */
> +	jmp int_ret_from_sys_call

Ok, dumb question: what is replacing that audit functionality now?
Or are we remaining with the single path down syscall_trace_leave()?
AFAICT, the intention is to direct everything to int_ret_from_sys_call
after the syscall has been done so that paths can get simplified. Which
is a good thing, AFAICT.

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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