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Date: Mon, 21 May 2018 23:39:46 -0700
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [dm-devel] [patch 4/4] dm-writecache: use new API for flushing
Message-ID: <20180522063946.GB8054@infradead.org>
References: <20180519052503.325953342@debian.vm>
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To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Mike Snitzer <msnitzer@redhat.com>, dm-devel@redhat.com, linux-nvdimm@lists.01.org
List-ID: <linux-nvdimm@lists.01.org>

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.
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