linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Date:	Mon, 27 Jan 2014 11:27:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath

On Sun, Jan 26, 2014 at 02:28:15PM -0800, Linus Torvalds wrote:
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 1e96c3628bf2..15b5953da958 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -658,30 +658,24 @@ sysret_check:
>  	/* Handle reschedules */
>  	/* edx:	work, edi: workmask */
>  sysret_careful:
>  	TRACE_IRQS_ON
>  	ENABLE_INTERRUPTS(CLBR_NONE)
> +	SAVE_REST
> +	FIXUP_TOP_OF_STACK %r11
> +	movq %rsp,%rdi
> +	movl %edx,%esi
> +	call syscall_exit_slowpath
> +	movl $_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT,%edi
> +	testl %eax,%eax
> +	jne sysret_using_iret
> +	addq $REST_SKIP,%rsp
> +	jmp sysret_check;

Obviously I don't particularly like the SAVE_REST/FIXUP_TOP_OF_STACK
being added to the reschedule path.

Can't we do as Al suggested earlier and have 2 slowpath calls, one
without PT_REGS and one with?

That said, yes its a nice cleanup, entry.S always hurts my brain.
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