linux-kernel - Re: [PATCH 00/13] dax, pmem: move cpu cache maintenance to libnvdimm

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Message-ID: <20170122184439.GA7603@lst.de>
Date:   Sun, 22 Jan 2017 19:44:39 +0100
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Brian Boylston <brian.boylston@....com>,
        Tony Luck <tony.luck@...el.com>, Jan Kara <jack@...e.cz>,
        Toshi Kani <toshi.kani@....com>,
        Mike Snitzer <snitzer@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Jeff Moyer <jmoyer@...hat.com>,
        Jens Axboe <axboe@...com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH 00/13] dax, pmem: move cpu cache maintenance to
        libnvdimm

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.