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

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Date:   Wed, 12 Oct 2016 01:25:33 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Dan Williams" <dan.j.williams@...el.com>
Cc:     "Stefano Stabellini" <stefano@...reto.com>,
        "Arnd Bergmann" <arnd@...db.de>, <andrew.cooper3@...rix.com>,
        "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 11.10.16 at 17:53, <dan.j.williams@...el.com> wrote:
> 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.

Despite me being the only one on the To list, I don't think the question
was really meant to be directed to me.

Jan