linux-kernel - RE: [PATCH v2] x86, pmem: fix broken __copy_user

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Date:   Mon, 10 Apr 2017 22:47:02 +0000
From:   "Kani, Toshimitsu" <toshi.kani@....com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: RE: [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.

Thanks,
-Toshi