From: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> Date: Wed, 25 May 2016 23:03:27 +0300 > On Wed, May 25, 2016 at 12:30:17PM -0700, David Miller wrote: >> From: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> >> Date: Tue, 24 May 2016 03:04:30 +0300 >> >> > +To clear that top halves, automatic wrappers are introduced. They clear all >> > +required registers before passing control to regular syscall handler. >> >> Why have one of these for every single compat system call, rather than >> simply clearing the top half of all of these registers unconditionally >> in the 32-bit system call trap before the system call is invoked? >> >> That's what we do on sparc64. >> >> And with that, you only need wrappers for the case where there needs >> to be proper sign extention of a 32-bit signed argument. > > It was discussed as one of possible solutions. The downside of it is > that we cannot pass 64-bit types (like off_t) in single register. Wrappers can be added for the cases where you'd like to do that. > The other downside is that we clear top halves for every single > syscall, and it looks excessive. So, from spark64 and s390 approaches > we choosed second. It's like 4 cpu cycles even on crappy sparc64 cpus which only dual issue. :) And that's a pretty low cost for the benefits if you ask me. -- 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
