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. Signed-off-by: Denys Vlasenko <[email protected]> CC: Josh Triplett <[email protected]> CC: Linus Torvalds <[email protected]> CC: Steven Rostedt <[email protected]> CC: Ingo Molnar <[email protected]> CC: Borislav Petkov <[email protected]> CC: "H. Peter Anvin" <[email protected]> CC: Andy Lutomirski <[email protected]> CC: Oleg Nesterov <[email protected]> CC: Frederic Weisbecker <[email protected]> CC: Alexei Starovoitov <[email protected]> CC: Will Drewry <[email protected]> CC: Kees Cook <[email protected]> CC: [email protected] CC: [email protected] --- This is a resend. There was a patch by Josh Triplett "x86: Opt into HAVE_COPY_THREAD_TLS, for both 32-bit and 64-bit" sent on May 11, which does the same thing as part of a bigger cleanup. He was supportive of this patch because of comments. He will simply have to drop one hunk from his patch. arch/x86/ia32/ia32entry.S | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S index 8e72256..0c302d0 100644 --- a/arch/x86/ia32/ia32entry.S +++ b/arch/x86/ia32/ia32entry.S @@ -567,11 +567,9 @@ GLOBAL(stub32_clone) * 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: + * We need to swap args here: */ - mov %r8, %rcx + xchg %r8, %rcx jmp ia32_ptregs_common ALIGN -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
