Re: [PATCH 3/4] x86: open-code register save/restore in trace_hardirqs thunks

[Andy Lutomirski <luto@amacapital.net>]

On Sat, Jan 10, 2015 at 2:00 PM, Denys Vlasenko <dvlasenk@redhat.com> 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.
>
> Take a page from thunk_32.S and use push/pop insns
> instead of movq, they are far shorter:
> 1 or 2 bytes versus 5, and no need for insns to adjust %rsp:
>
>    text    data     bss     dec     hex filename
>     333      40       0     373     175 thunk_64_movq.o
>     104      40       0     144      90 thunk_64_push_pop.o
>
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Oleg Nesterov <oleg@redhat.com>
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Borislav Petkov <bp@alien8.de>
> CC: Andy Lutomirski <luto@amacapital.net>
> CC: Frederic Weisbecker <fweisbec@gmail.com>
> CC: X86 ML <x86@kernel.org>
> CC: Alexei Starovoitov <ast@plumgrid.com>
> CC: Will Drewry <wad@chromium.org>
> CC: Kees Cook <keescook@chromium.org>
> CC: linux-kernel@vger.kernel.org
> ---
>  arch/x86/lib/thunk_64.S | 46 ++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 42 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/lib/thunk_64.S b/arch/x86/lib/thunk_64.S
> index b30b5eb..8ec443a 100644
> --- a/arch/x86/lib/thunk_64.S
> +++ b/arch/x86/lib/thunk_64.S
> @@ -17,9 +17,27 @@
>         CFI_STARTPROC
>
>         /* this one pushes 9 elems, the next one would be %rIP */
> -       SAVE_ARGS
> +       pushq_cfi %rdi
> +       CFI_REL_OFFSET rdi, 0
> +       pushq_cfi %rsi
> +       CFI_REL_OFFSET rsi, 0
> +       pushq_cfi %rdx
> +       CFI_REL_OFFSET rdx, 0
> +       pushq_cfi %rcx
> +       CFI_REL_OFFSET rcx, 0
> +       pushq_cfi %rax
> +       CFI_REL_OFFSET rax, 0
> +       pushq_cfi %r8
> +       CFI_REL_OFFSET r8, 0
> +       pushq_cfi %r9
> +       CFI_REL_OFFSET r9, 0
> +       pushq_cfi %r10
> +       CFI_REL_OFFSET r10, 0
> +       pushq_cfi %r11
> +       CFI_REL_OFFSET r11, 0
>
>         .if \put_ret_addr_in_rdi
> +       /* 9*8(%rsp) is return addr on stack */
>         movq_cfi_restore 9*8, rdi
>         .endif
>
> @@ -45,11 +63,31 @@
>  #endif
>  #endif
>
> -       /* SAVE_ARGS below is used only for the .cfi directives it contains. */
> +#if defined(CONFIG_TRACE_IRQFLAGS) \
> + || defined(CONFIG_DEBUG_LOCK_ALLOC) \
> + || defined(CONFIG_PREEMPT)
>         CFI_STARTPROC
> -       SAVE_ARGS
> +       CFI_ADJUST_CFA_OFFSET 9*8
>  restore:
> -       RESTORE_ARGS
> +       popq_cfi %r11
> +       CFI_RESTORE r11
> +       popq_cfi %r10
> +       CFI_RESTORE r10
> +       popq_cfi %r9
> +       CFI_RESTORE r9
> +       popq_cfi %r8
> +       CFI_RESTORE r8
> +       popq_cfi %rax
> +       CFI_RESTORE rax
> +       popq_cfi %rcx
> +       CFI_RESTORE rcx
> +       popq_cfi %rdx
> +       CFI_RESTORE rdx
> +       popq_cfi %rsi
> +       CFI_RESTORE rsi
> +       popq_cfi %rdi
> +       CFI_RESTORE rdi
>         ret
>         CFI_ENDPROC
>         _ASM_NOKPROBE(restore)
> +#endif


Applied.  I'll deal with the cleanup as a followup tomorrow.