On Wed, May 13, 2015 at 09:02:41AM -0700, Linus Torvalds wrote:
> The nice thing about using "rep movsb" for the user copy is that not
> only is it fairly close to optimal (for non-constant sizes) on newer
> Intel CPU's, but the fixup is also trivial. So we really should inline
> it. Just look at it: the copy_user_enhanced_fast_string function is
> literally just three 2-byte instructions right now:
>
> mov %edx,%ecx
> rep movsb
> xor %eax,%eax
>
> and the rest is just the exception table thing.
Yeah, so I thought about it for a while and yeah, those labels there
would be a problem. Because you have this:
mov %edx,%ecx
1: rep movsb
xor %eax,%eax
and _ASM_EXTABLE adds the .fixup section entry in the form of relative
offsets.
So I *think* it would work if we make the REP;STOSB case the default
one, i.e. those insns get issued during build. Then the labels will be
fine and all is good.
When they have to get patched probably with a CALL to the other
variants, the label 1: above (or rather the fixup entry) will point to
the newly patched instruction which, if it faults, might get fixed up
erroneously.
Hmm, let me give it a try - I'll have a better idea after I've done it.
> (And yes, there's the STAC/CLAC thing around it, but I think that
> should just be moved into _copy_from/to_user() too, since *all* of the
> copy_user_generic() cases need it).
Yeah.
> Yeah, yeah, we'd still do the double call thing for the more complex
> cases of the unrolled copy loop or the "movsq + tail" cases, but those
> are at least big enough that it makes sense. And they are presumably
> getting less common anyway.
Right, so we can avoid the first CALL if I inline copy_user_generic()
which practically inlines the alternative directly.
Lemme play with it a little...
Thanks.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
--
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
