Re: [patch 4/4] dm-writecache: use new API for flushing

[Mike Snitzer <snitzer@redhat.com>]

On Tue, May 22 2018 at  2:39am -0400,
Christoph Hellwig <hch@infradead.org> wrote:

> On Sat, May 19, 2018 at 07:25:07AM +0200, Mikulas Patocka wrote:
> > Use new API for flushing persistent memory.
> 
> The sentence doesnt make much sense.  'A new API', 'A better
> abstraction' maybe?
> 
> > 
> > The problem is this:
> > * on X86-64, non-temporal stores have the best performance
> > * ARM64 doesn't have non-temporal stores, so we must flush cache. We
> >   should flush cache as late as possible, because it performs better this
> >   way.
> > 
> > We introduce functions pmem_memcpy, pmem_flush and pmem_commit. To commit
> > data persistently, all three functions must be called.
> > 
> > The macro pmem_assign may be used instead of pmem_memcpy. pmem_assign
> > (unlike pmem_memcpy) guarantees that 8-byte values are written atomically.
> > 
> > On X86, pmem_memcpy is memcpy_flushcache, pmem_flush is empty and
> > pmem_commit is wmb.
> > 
> > On ARM64, pmem_memcpy is memcpy, pmem_flush is arch_wb_cache_pmem and
> > pmem_commit is empty.
> 
> All these should be provided by the pmem layer, and be properly
> documented.  And be sorted before adding your new target that uses
> them.

I don't see that as a hard requirement.  Mikulas did the work to figure
out what is more optimal on x86_64 vs amd64.  It makes a difference for
his target and that is sufficient to carry it locally until/when it is
either elevated to pmem.

We cannot even get x86 and swait maintainers to reply to repeat requests
for review.  Stacking up further deps on pmem isn't high on my list.

Mike
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