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SubjectRE: [PATCH v2] x86, pmem: fix broken __copy_user_nocache cache-bypass assumptions
> >> > The clflush here flushes for the cacheline size.  So, we do not need to
> flush
> >> > the same cacheline again when the unaligned tail is in the same line.
> >>
> >> Ok, makes sense. Last question, can't we reduce the check to be:
> >>
> >> if ((bytes > flushed) && ((bytes - flushed) & 3))
> >>
> >> ...since if 'bytes' was 4-byte aligned we would have performed
> >> non-temporal stores.
> >
> > That is not documented behavior of copy_user_nocache, but as long as the
> pmem
> > version of copy_user_nocache follows the same implemented behavior, yes,
> that
> > works.
> Hmm, sorry this comment confuses me, I'm only referring to the current
> version of __copy_user_nocache not the new pmem version. The way I
> read the current code we only ever jump to the cached copy loop
> (.L_1b_cache_copy_loop) if the trailing byte-count is 4-byte
> misaligned.

Yes, you are right and that's how the code is implemented. I added this trailing
4-byte handling for the >=8B case, which is shared with <8B case, since it was
easy to do. But I considered it a bonus. This function also needs to handle
4B-aligned destination if it is to state that it handles 4B alignment for the >=8B
case as well. Otherwise, it's inconsistent. Since I did not see much point of supporting
such case, I simply documented in the Note that 8 byte alignment is required for
the >=8B case.


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