> > > The cost is pretty trivial though. See kernel/compat_wrapper.o: > > > COMPAT_SYSCALL_WRAP2(creat, const char __user *, pathname, umode_t, mode); > > > 0: a9bf7bfd stp x29, x30, [sp,#-16]! > > > 4: 910003fd mov x29, sp > > > 8: 2a0003e0 mov w0, w0 > > > c: 94000000 bl 0 <sys_creat> > > > 10: a8c17bfd ldp x29, x30, [sp],#16 > > > 14: d65f03c0 ret > > > > I would say the above could be more expensive than 8 movs (16 bytes to > > write, read, a branch and a ret). You can also add the I-cache locality, > > having wrappers for each syscalls instead of a single place for zeroing > > the upper half (where no other wrapper is necessary). > > > > Can we trick the compiler into doing a tail call optimisation. This > > could have simply been: > > > > COMPAT_SYSCALL_WRAP2(creat, ...): > > mov w0, w0 > > b <sys_creat> > > What you talk about was in my initial version. But Heiko insisted on having all > wrappers together. > http://www.spinics.net/lists/linux-s390/msg11593.html > > Grep your email for discussion. I think Catalin's question was more about why there is even a stack frame generated. It looks like it is not necessary. I did ask this too a couple of months ago, when we discussed this. > > > > Cost wise, this seems like it all cancels out in the end, but what > > > > do I know? > > > > > > I think you know something, and I also think Heiko and other s390 guys > > > know something as well. So I'd like to listen their arguments here. If it comes to 64 bit arguments for compat system calls: s390 also has an x32-like ABI extension which allows user space to use full 64 bit registers. As far as I know hardly anybody ever made use of that. However even if that would be widely used, to me it wouldn't make sense to add new compat system calls which allow 64 bit arguments, simply because something like c = (u32)a | (u64)b << 32; can be done with a single 1-cycle instruction. It's just not worth the extra effort to maintain additional system call variants. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
