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Message-ID: <tip-7a5a9824c18f93415944c997dc6bb8eecfddd2e7@git.kernel.org> Date: Sun, 7 Jun 2015 01:32:49 -0700 From: tip-bot for Denys Vlasenko <tipbot@...or.com> To: linux-tip-commits@...r.kernel.org Cc: linux-kernel@...r.kernel.org, rostedt@...dmis.org, wad@...omium.org, akpm@...ux-foundation.org, tglx@...utronix.de, brgerst@...il.com, josh@...htriplett.org, hpa@...or.com, torvalds@...ux-foundation.org, dvlasenk@...hat.com, luto@...capital.net, keescook@...omium.org, mingo@...nel.org, fweisbec@...il.com, peterz@...radead.org, ast@...mgrid.com, bp@...en8.de, oleg@...hat.com Subject: [tip:x86/asm] x86/asm/entry/32: Remove unnecessary optimization in stub32_clone Commit-ID: 7a5a9824c18f93415944c997dc6bb8eecfddd2e7 Gitweb: http://git.kernel.org/tip/7a5a9824c18f93415944c997dc6bb8eecfddd2e7 Author: Denys Vlasenko <dvlasenk@...hat.com> AuthorDate: Wed, 3 Jun 2015 15:58:50 +0200 Committer: Ingo Molnar <mingo@...nel.org> CommitDate: Fri, 5 Jun 2015 13:41:28 +0200 x86/asm/entry/32: Remove unnecessary optimization in stub32_clone Really swap arguments #4 and #5 in stub32_clone instead of "optimizing" it into a move. Yes, tls_val is currently unused. Yes, on some CPUs XCHG is a little bit more expensive than MOV. But a cycle or two on an expensive syscall like clone() is way below noise floor, and this optimization is simply not worth the obfuscation of logic. [ There's also ongoing work on the clone() ABI by Josh Triplett that will depend on this change later on. ] Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com> Cc: Alexei Starovoitov <ast@...mgrid.com> Cc: Andrew Morton <akpm@...ux-foundation.org> Cc: Andy Lutomirski <luto@...capital.net> Cc: Borislav Petkov <bp@...en8.de> Cc: Brian Gerst <brgerst@...il.com> Cc: Frederic Weisbecker <fweisbec@...il.com> Cc: H. Peter Anvin <hpa@...or.com> Cc: Josh Triplett <josh@...htriplett.org> Cc: Kees Cook <keescook@...omium.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Oleg Nesterov <oleg@...hat.com> Cc: Peter Zijlstra <peterz@...radead.org> Cc: Steven Rostedt <rostedt@...dmis.org> Cc: Thomas Gleixner <tglx@...utronix.de> Cc: Will Drewry <wad@...omium.org> Link: http://lkml.kernel.org/r/1433339930-20880-2-git-send-email-dvlasenk@redhat.com Signed-off-by: Ingo Molnar <mingo@...nel.org> --- arch/x86/entry/ia32entry.S | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) diff --git a/arch/x86/entry/ia32entry.S b/arch/x86/entry/ia32entry.S index d0c7b28..9558dac 100644 --- a/arch/x86/entry/ia32entry.S +++ b/arch/x86/entry/ia32entry.S @@ -529,14 +529,13 @@ GLOBAL(\label) GLOBAL(stub32_clone) leaq sys_clone(%rip), %rax /* - * 32-bit clone API is clone(..., int tls_val, int *child_tidptr). - * 64-bit clone API is clone(..., int *child_tidptr, int tls_val). - * Native 64-bit kernel's sys_clone() implements the latter. - * We need to swap args here. But since tls_val is in fact ignored - * by sys_clone(), we can get away with an assignment - * (arg4 = arg5) instead of a full swap: + * The 32-bit clone ABI is: clone(..., int tls_val, int *child_tidptr). + * The 64-bit clone ABI is: clone(..., int *child_tidptr, int tls_val). + * + * The native 64-bit kernel's sys_clone() implements the latter, + * so we need to swap arguments here before calling it: */ - mov %r8, %rcx + xchg %r8, %rcx jmp ia32_ptregs_common ALIGN -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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