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 12:28:56 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Jan Beulich <jbeulich@...e.com>, Juergen Gross <JGross@...e.com>,
        Haozhong Zhang <haozhong.zhang@...el.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        andrew.cooper3@...rix.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stefano Stabellini <stefano@...reto.com>,
        David Vrabel <david.vrabel@...rix.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        xen-devel@...ts.xenproject.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [Xen-devel] [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for Xen

On Tue, Oct 11, 2016 at 11:33 AM, Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
> On Tue, Oct 11, 2016 at 10:51:19AM -0700, Dan Williams wrote:
[..]
>> Right, but why does the libnvdimm core need to know about this
>> specific Xen reservation?  For example, if Xen wants some in-kernel
>
> Let me turn this around - why does the libnvdimm core need to know about
> Linux specific parts? Shouldn't this be OS agnostic, so that FreeBSD
> for example can also poke a hole in this and fill it with its
> OS-management meta-data?

Specifically the core needs to know so that it can answer the Linux
specific question of whether the pfn returned by ->direct_access() has
a corresponding struct page or not. It's tied to the lifetime of the
device and the usage of the reservation needs to be coordinated
against the references of those pages.  If FreeBSD decides it needs to
reserve "struct page" capacity at the start of the device, I would
hope that it reuses the same on-device info block that Linux is using
and not create a new "FreeBSD-mode" device type.

To be honest I do not yet understand what metadata Xen wants to store
in the device, but it seems the producer and consumer of that metadata
is Xen itself and not the wider Linux kernel as is the case with
struct page.  Can you fill me in on what problem Xen solves with this
reservation?