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Date: Sun, 22 Jan 2017 19:44:39 +0100
From: Christoph Hellwig <hch@lst.de>
To: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
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Subject: Re: [PATCH 00/13] dax, pmem: move cpu cache maintenance to
        libnvdimm
Message-ID: <20170122184439.GA7603@lst.de>
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On Sun, Jan 22, 2017 at 06:39:28PM +0000, Matthew Wilcox wrote:
> Two guests on the same physical machine (or a guest and a host) have access to the same set of physical addresses.  This might be an NV-DIMM, or it might just be DRAM (for the purposes of reducing guest overhead).  The network filesystem has been enhanced with a call to allow the client to ask the server "What is the physical address for this range of bytes in this file?"
> 
> We don't want to use the guest pagecache here.  That's antithetical to the second usage, and it's inefficient for the first usage.

And the answer is that you need a dax device for whatever memoery exposed
in this way, as it needs to show up in the memory map for example.