Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 3 Jul 2020 14:10:08 -0700 | Subject | Re: objtool clac/stac handling change.. |
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On Fri, Jul 3, 2020 at 2:02 PM Al Viro <viro@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.
Yeah, the "stac" instruction isn't hugely fast, and serializes the pipeline, so it's a nasty 20 cycles or something.
But for chissake, this (a) happens approximately never (b) is after a fault that took a thousand cycles
so the trivial thing to do is to just say "yeah, you need to add the STAC when your optimistic thing failed and you have to fall back to the byte-at-a-time tail case".
It's particularly trivial since objtool would statically find all these places, since it would warn about the ASM_CLAC without a STAC (that's assuming Josh's patch, of course).
Linus
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