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

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Date:   Mon, 10 Apr 2017 21:45:47 +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

> On Mon, Apr 10, 2017 at 2:28 PM, Kani, Toshimitsu <toshi.kani@....com>
> wrote:
> >> > Thanks for the update.  I think the alignment check should be based on
> >> > the following note in copy_user_nocache.
> >> >
> >> >  * Note: Cached memory copy is used when destination or size is not
> >> >  * naturally aligned. That is:
> >> >  *  - Require 8-byte alignment when size is 8 bytes or larger.
> >> >  *  - Require 4-byte alignment when size is 4 bytes.
> >> >
> >> > So, I think the code may be something like this.  I also made the following
> >> changes:
> >>
> >> Thanks!
> >>
> >> >  - Mask with 7, not 8.
> >>
> >> Yes, good catch.
> >>
> >> >  - ALIGN with cacheline size, instead of 8.
> >> >  - Add (bytes > flushed) test since calculation with unsigned long still
> results
> >> in a negative
> >> >    value (as a positive value).
> >> >
> >> >         if (bytes < 8) {
> >> >                 if ((dest & 3) || (bytes != 4))
> >> >                         arch_wb_cache_pmem(addr, 1);
> >> >         } else {
> >> >                 if (dest & 7) {
> >> >                         dest = ALIGN(dest, boot_cpu_data.x86_clflush_size);
> >>
> >> Why align the destination to the next cacheline? As far as I can see
> >> the ALIGN_DESTINATION macro in arch/x86/include/asm/asm.h only aligns
> >> to the next 8-byte boundary.
> >
> > 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.

> Can I add your Signed-off-by: on v3?

Sure.

Thanks,
-Toshi