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

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Date:   Mon, 10 Oct 2016 09:24:26 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        xen-devel@...ts.xenproject.org,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        David Vrabel <david.vrabel@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Arnd Bergmann <arnd@...db.de>,
        Stefano Stabellini <stefano@...reto.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for Xen

On Sun, Oct 9, 2016 at 11:32 PM, Haozhong Zhang
<haozhong.zhang@...el.com> wrote:
> On 10/09/16 20:45, Dan Williams wrote:
>> On Sun, Oct 9, 2016 at 5:35 PM, Haozhong Zhang <haozhong.zhang@...el.com> wrote:
>> > Overview
>> > ========
>> > This RFC kernel patch series along with corresponding patch series of
>> > Xen, QEMU and ndctl implements Xen vNVDIMM, which can map the host
>> > NVDIMM devices to Xen HVM domU as vNVDIMM devices.
>> >
>> > Xen hypervisor does not include an NVDIMM driver, so it needs the
>> > assistance from the driver in Dom0 Linux kernel to manage NVDIMM
>> > devices. We currently only supports NVDIMM devices in pmem mode.
>> >
>> > Design and Implementation
>> > =========================
>> > The complete design can be found at
>> >   https://lists.xenproject.org/archives/html/xen-devel/2016-07/msg01921.html.
>>
>> The KVM enabling for persistent memory does not need this support from
>> the kernel, and as far as I can see neither does Xen. If the
>> hypervisor needs to reserve some space it can simply trim the amount
>> that it hands to the guest.
>>
>
> Xen does not have the NVDIMM driver, so it cannot operate on NVDIMM
> devices by itself. Instead it relies on the driver in Dom0 Linux to
> probe NVDIMM and make the reservation.

I'm missing something because the design document talks about mmap'ing
files on a DAX filesystem.  So, I'm assuming it is similar to the KVM
NVDIMM virtualization case where an mmap range in dom0 is translated
into a guest physical range.  The suggestion is to reserve some memory
out of that mapping rather than introduce a new info block /
reservation type to the sub-system.