linux-kernel - Re: [Xen-devel] [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for Xen

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Date:   Tue, 11 Oct 2016 08:53:33 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     andrew.cooper3@...rix.com,
        Stefano Stabellini <stefano@...reto.com>,
        Arnd Bergmann <arnd@...db.de>,
        David Vrabel <david.vrabel@...rix.com>,
        Haozhong Zhang <haozhong.zhang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        xen-devel@...ts.xenproject.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <JGross@...e.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for Xen

On Tue, Oct 11, 2016 at 6:08 AM, Jan Beulich <jbeulich@...e.com> wrote:
>>>> Andrew Cooper <andrew.cooper3@...rix.com> 10/10/16 6:44 PM >>>
>>On 10/10/16 01:35, Haozhong Zhang wrote:
>>> Xen hypervisor needs assistance from Dom0 Linux kernel for following tasks:
>>> 1) Reserve an area on NVDIMM devices for Xen hypervisor to place
>>>    memory management data structures, i.e. frame table and M2P table.
>>> 2) Report SPA ranges of NVDIMM devices and the reserved area to Xen
>>>    hypervisor.
>>
>>However, I can't see any justification for 1).  Dom0 should not be
>>involved in Xen's management of its own frame table and m2p.  The mfns
>>making up the pmem/pblk regions should be treated just like any other
>>MMIO regions, and be handed wholesale to dom0 by default.
>
> That precludes the use as RAM extension, and I thought earlier rounds of
> discussion had got everyone in agreement that at least for the pmem case
> we will need some control data in Xen.

The missing piece for me is why this reservation for control data
needs to be done in the libnvdimm core?  I would expect that any dax
capable file could be mapped and made available to a guest.  This
includes /dev/ramX devices that are dax capable, but are external to
the libnvdimm sub-system.