linux-kernel - Re: [PATCH 2/7] x86: entry.S: simplify optimistic SYSRET

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Message-ID: <CALCETrUy7ewDhPSLK+79xG8VhfPKxUiWgvaHvcTm-0bfi3viqw@mail.gmail.com>
Date:	Tue, 24 Feb 2015 16:04:55 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/7] x86: entry.S: simplify optimistic SYSRET

On Tue, Feb 24, 2015 at 4:00 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> Avoid redundant load of %r11 (it is already loaded a few instructions before).
> Do not needlessly increment %rsp - we are going to return to userspace
> via SYSRET, this insn doesn't use stack for return.
>
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ingo Molnar <mingo@...nel.org>
> CC: Borislav Petkov <bp@...en8.de>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org
> ---
>  arch/x86/include/asm/calling.h | 3 +++
>  arch/x86/kernel/entry_64.S     | 5 ++---
>  2 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
> index 3374235..f1a962f 100644
> --- a/arch/x86/include/asm/calling.h
> +++ b/arch/x86/include/asm/calling.h
> @@ -176,6 +176,9 @@ For 32-bit we have the following conventions - kernel is built with
>         .macro RESTORE_C_REGS_EXCEPT_RCX
>         RESTORE_C_REGS_HELPER 1,0,1,1,1
>         .endm
> +       .macro RESTORE_C_REGS_EXCEPT_R11
> +       RESTORE_C_REGS_HELPER 1,1,0,1,1
> +       .endm
>         .macro RESTORE_RSI_RDI
>         RESTORE_C_REGS_HELPER 0,0,0,0,0
>         .endm
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 4b3f3c1..e5e0a55 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -756,9 +756,8 @@ retint_swapgs:              /* return to user-space */
>          */
>  irq_return_via_sysret:
>         CFI_REMEMBER_STATE
> -       RESTORE_C_REGS
> -       REMOVE_PT_GPREGS_FROM_STACK 8
> -       movq (RSP-RIP)(%rsp),%rsp
> +       RESTORE_C_REGS_EXCEPT_R11
> +       movq RSP(%rsp),%rsp

I think this needs a comment.

--Andy

>         USERGS_SYSRET64
>         CFI_RESTORE_STATE
>
> --
> 1.8.1.4
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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