[dm-devel] [PATCH] x86: optimize memcpy_flushcache

[Yigal Korman]

On Wed, Jun 27, 2018 at 4:03 PM, Dan Williams <dan.j.williams at intel.com> wrote:
> On Wed, Jun 27, 2018 at 4:23 AM, Yigal Korman <yigal at plexistor.com> wrote:
>> Hi,
>> I'm a bit late on this but I have a question about the original patch -
>> I thought that in order for movnt (movntil, movntiq) to push the data
>> into the persistency domain (ADR),
>> one must work with length that is multiple of cacheline size,
>> otherwise the write-combine buffers remain partially
>> filled and you need to commit them with a fence (sfence) - which ruins
>> the whole performance gain you got here.
>> Am I wrong, are the write-combine buffers are part of the ADR domain
>> or something?
>
> The intent is to allow a batch of memcpy_flushcache() calls followed
> by a single sfence. Specifying a multiple of a cacheline size does not
> necessarily help as sfence is still needed to make sure that the movnt
> result has reached the ADR-safe domain.

Oh, right, I see that dm-writecache calls writecache_commit_flushed
which in turn calls wmb().
I keep confusing *_nocache (i.e. copy_user_nocache) that includes
sfence and *_flushcache (i.e. memcpy_flushcache) that doesn't.
Thanks for the clear up.