linux-kernel - Re: [PATCH 07/18] x86/asm/64: Merge the fast and slow SYSRET paths

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Message-ID: <20171027201125.GD12039@nazgul.tnic>
Date:   Fri, 27 Oct 2017 22:11:25 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 07/18] x86/asm/64: Merge the fast and slow SYSRET paths

On Thu, Oct 26, 2017 at 01:26:39AM -0700, Andy Lutomirski wrote:
> They did almost the same thing.  Remove a bunch of pointless
> instructions (mostly hidden in macros) and reduce cognitive load by
> merging them.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/entry_64.S | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 9dafafa3e0ec..c855ee91a3a5 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -220,10 +220,9 @@ entry_SYSCALL_64_fastpath:
>  	TRACE_IRQS_ON		/* user mode is traced as IRQs on */
>  	movq	RIP(%rsp), %rcx
>  	movq	EFLAGS(%rsp), %r11
> -	RESTORE_C_REGS_EXCEPT_RCX_R11
> -	movq	RSP(%rsp), %rsp
> +	addq	$6*8, %rsp	/* skip extra regs -- they were preserved */
>  	UNWIND_HINT_EMPTY
> -	USERGS_SYSRET64
> +	jmp	.Lpop_c_regs_except_rcx_r11_and_sysret
>  
>  1:
>  	/*
> @@ -315,6 +314,7 @@ syscall_return_via_sysret:
>  	/* rcx and r11 are already restored (see code above) */
>  	UNWIND_HINT_EMPTY
>  	POP_EXTRA_REGS
> +.Lpop_c_regs_except_rcx_r11_and_sysret:

.Lpop_regs_sysret I guess.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--