[PATCH 2/7 v2] x86: entry.S: simplify optimistic SYSRET

[PATCH 2/7 v2] x86: entry.S: simplify optimistic SYSRET

[Denys Vlasenko]

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.

Changes since v1: added a comment

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
 arch/x86/include/asm/calling.h | 3 +++
 arch/x86/kernel/entry_64.S     | 6 +++---
 2 files changed, 6 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..4f113c8 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -756,9 +756,9 @@ 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
+	/* r11 is already restored (see code above) */
+	RESTORE_C_REGS_EXCEPT_R11
+	movq RSP(%rsp),%rsp
 	USERGS_SYSRET64
 	CFI_RESTORE_STATE
 
-- 
1.8.1.4

Re: [PATCH 2/7 v2] x86: entry.S: simplify optimistic SYSRET

[Andy Lutomirski]

On Tue, Feb 24, 2015 at 4:30 PM, Denys Vlasenko <dvlasenk@redhat.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.

Applied.

>
> Changes since v1: added a comment
>
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Borislav Petkov <bp@alien8.de>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Oleg Nesterov <oleg@redhat.com>
> CC: Frederic Weisbecker <fweisbec@gmail.com>
> CC: Alexei Starovoitov <ast@plumgrid.com>
> CC: Will Drewry <wad@chromium.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: x86@kernel.org
> CC: linux-kernel@vger.kernel.org
> ---
>  arch/x86/include/asm/calling.h | 3 +++
>  arch/x86/kernel/entry_64.S     | 6 +++---
>  2 files changed, 6 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..4f113c8 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -756,9 +756,9 @@ 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
> +       /* r11 is already restored (see code above) */
> +       RESTORE_C_REGS_EXCEPT_R11
> +       movq RSP(%rsp),%rsp
>         USERGS_SYSRET64
>         CFI_RESTORE_STATE
>
> --
> 1.8.1.4
>



-- 
Andy Lutomirski
AMA Capital Management, LLC