Re: [PATCH v2] x86, pmem: fix broken __copy_user_nocache cache-bypass assumptions

[Dan Williams]

On Mon, Apr 10, 2017 at 2:28 PM, Kani, Toshimitsu <toshi.kani@xxxxxxx> 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.

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