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

[Andy Lutomirski]

On Tue, Feb 24, 2015 at 4:00 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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@xxxxxxxxxx>
> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> CC: Will Drewry <wad@xxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: x86@xxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  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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