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 09:39:30 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Dan Williams" <dan.j.williams@...el.com>,
        "Haozhong Zhang" <haozhong.zhang@...el.com>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:     "Stefano Stabellini" <stefano@...reto.com>,
        "Arnd Bergmann" <arnd@...db.de>, <andrew.cooper3@...rix.com>,
        "David Vrabel" <david.vrabel@...rix.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Xiao Guangrong" <guangrong.xiao@...ux.intel.com>,
        "Ross Zwisler" <ross.zwisler@...ux.intel.com>,
        <xen-devel@...ts.xenproject.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.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 12.10.16 at 16:58, <haozhong.zhang@...el.com> wrote:
> On 10/12/16 05:32 -0600, Jan Beulich wrote:
>>>>> On 12.10.16 at 12:33, <haozhong.zhang@...el.com> wrote:
>>> The layout is shown as the following diagram.
>>>
>>> +---------------+-----------+-------+----------+--------------+
>>> | whatever used | Partition | Super | Reserved | /dev/pmem0p1 |
>>> |  by kernel    |   Table   | Block | for Xen  |              |
>>> +---------------+-----------+-------+----------+--------------+
>>>                 \_____________________ _______________________/
>>> 		                      V
>>> 				 /dev/pmem0
>>
>>I have to admit that I dislike this, for not being OS-agnostic.
>>Neither should there be any Xen-specific region, nor should the
>>"whatever used by kernel" one be restricted to just Linux. What
>>I could see is an OS-reserved area ahead of the partition table,
>>the exact usage of which depends on which OS is currently
>>running (and in the Xen case this might be both Xen _and_ the
>>Dom0 kernel, arbitrated by a tbd protocol). After all, when
>>running under Xen, the Dom0 may not have a need for as much
>>control data as it has when running on bare hardware, for it
>>controlling less (if any) of the actual memory ranges when Xen
>>is present.
>>
> 
> Isn't this OS-reserved area still not OS-agnostic, as it requires OS
> to know where the reserved area is?  Or do you mean it's not if it's
> defined by a protocol that is accepted by all OSes?

The latter - we clearly won't get away without some agreement on
where to retrieve position and size of this area. I was simply
assuming that such a protocol already exists.

Jan