linux-kernel - Re: [PATCH 3/4] x86: open-code register save/restore in trace

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Date:	Fri, 9 Jan 2015 11:55:41 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 3/4] x86: open-code register save/restore in
 trace_hardirqs thunks

On Thu, Jan 08, 2015 at 05:25:14PM +0100, Denys Vlasenko wrote:
> This is a preparatory patch for change in "struct pt_regs"
> handling in entry_64.S.
> 
> trace_hardirqs thunks were (ab)using a part of pt_regs
> handling code, namely SAVE_ARGS/RESTORE_ARGS macros,
> to save/restore registers across C function calls.
> 
> Since SAVE_ARGS is going to be changed, open-code
> register saving/restoring here.
> 
> Incidentally, this removes a bit of dead code:
> one SAVE_ARGS was used just to emit a CFI annotation,
> but it also generated unreachable assembly insns.
> 
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: X86 ML <x86@...nel.org>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: linux-kernel@...r.kernel.org
> ---
>  arch/x86/lib/thunk_64.S | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/lib/thunk_64.S b/arch/x86/lib/thunk_64.S
> index b30b5eb..03a3883 100644
> --- a/arch/x86/lib/thunk_64.S
> +++ b/arch/x86/lib/thunk_64.S
> @@ -16,10 +16,20 @@
>  \name:
>  	CFI_STARTPROC
>  
> -	/* this one pushes 9 elems, the next one would be %rIP */
> -	SAVE_ARGS
> +	subq  $9*8, %rsp
> +	CFI_ADJUST_CFA_OFFSET 9*8
> +	movq_cfi r11, 0*8
> +	movq_cfi r10, 1*8
> +	movq_cfi r9,  2*8
> +	movq_cfi r8,  3*8
> +	movq_cfi rax, 4*8
> +	movq_cfi rcx, 5*8
> +	movq_cfi rdx, 6*8
> +	movq_cfi rsi, 7*8
> +	movq_cfi rdi, 8*8
>  
>  	.if \put_ret_addr_in_rdi
> +	/* 9*8(%rsp) is return addr on stack */
>  	movq_cfi_restore 9*8, rdi
>  	.endif
>  
> @@ -45,11 +55,20 @@
>  #endif
>  #endif
>  
> -	/* SAVE_ARGS below is used only for the .cfi directives it contains. */
>  	CFI_STARTPROC
> -	SAVE_ARGS
> +	CFI_ADJUST_CFA_OFFSET 9*8
>  restore:
> -	RESTORE_ARGS
> +	movq_cfi_restore 0*8, r11
> +	movq_cfi_restore 1*8, r10
> +	movq_cfi_restore 2*8, r9
> +	movq_cfi_restore 3*8, r8
> +	movq_cfi_restore 4*8, rax
> +	movq_cfi_restore 5*8, rcx
> +	movq_cfi_restore 6*8, rdx
> +	movq_cfi_restore 7*8, rsi
> +	movq_cfi_restore 8*8, rdi
> +	addq 9*8, %rsp
> +	CFI_ADJUST_CFA_OFFSET -9*8

The only nitpick I'd have with this is can we keep the register
saving/restoring order in the code the same as in the SAVE_/RESTORE_ARGS
macros?

SAVE_ARGS starts with the highest offset 9*8, rdi and ends at 0*8 and
r11 and RESTORE_ARGS does that in reverse.

Also, can you post the struct pt_regs change too so that we know where
this is going?

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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