On Fri, Jul 03, 2020 at 10:59:22PM +0100, Al Viro wrote: > On Fri, Jul 03, 2020 at 02:10:08PM -0700, Linus Torvalds wrote: > > On Fri, Jul 3, 2020 at 2:02 PM Al Viro <v...@zeniv.linux.org.uk> wrote: > > > > > > Actually, for more serious problem consider arch/x86/lib/copy_user_64.S > > > > What? No. > > > > > In case of an unhandled fault on attempt to read an (unaligned) word, > > > the damn thing falls back to this: > > > SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail) > > > movl %edx,%ecx > > > 1: rep movsb > > > 2: mov %ecx,%eax > > > ASM_CLAC > > > ret > > > > > > _ASM_EXTABLE_UA(1b, 2b) > > > SYM_CODE_END(.Lcopy_user_handle_tail) > > > > In the case of "we did an unaligned word at the end of a page, we took > > a fault, and now we have to start all over", the _least_ of our > > problems is that part of "starting over" would now imply doing a > > "stac" again. > > What do you mean, start over? It's picking a few remaining bytes out > of that word, *not* redoing the entire thing.
I'm _not_ saying that it's a performance-critical place, in case that's not obvious - just trying to head off potential confusion re what that code is doing.
