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 13:52:58 +0800
From:   Haozhong Zhang <haozhong.zhang@...el.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     <linux-nvdimm@...ts.01.org>, <xen-devel@...ts.xenproject.org>,
        Juergen Gross <jgross@...e.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        <linux-kernel@...r.kernel.org>,
        Stefano Stabellini <stefano@...reto.com>,
        David Vrabel <david.vrabel@...rix.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [Xen-devel] [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for
 Xen

On 10/10/16 17:43, Andrew Cooper wrote:
> On 10/10/16 01:35, Haozhong Zhang 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.
> >
> > All patch series can be found at
> >   Xen:          https://github.com/hzzhan9/xen.git nvdimm-rfc-v1
> >   QEMU:         https://github.com/hzzhan9/qemu.git xen-nvdimm-rfc-v1
> >   Linux kernel: https://github.com/hzzhan9/nvdimm.git xen-nvdimm-rfc-v1
> >   ndctl:        https://github.com/hzzhan9/ndctl.git pfn-xen-rfc-v1
> >
> > 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.
> 
> Please can we take a step back here before diving down a rabbit hole.
> 
> 
> How do pblk/pmem regions appear in the E820 map at boot?  At the very
> least, I would expect at least a large reserved region.

ACPI specification does not require them to appear in E820, though
it defines E820 type-7 for persistent memory.

> 
> Is the MFN information (SPA in your terminology, so far as I can tell)
> available in any static APCI tables, or are they only available as a
> result of executing AML methods?
>

For NVDIMM devices already plugged at power on, their MFN information
can be got from NFIT table. However, MFN information for hotplugged
NVDIMM devices should be got via AML _FIT method, so point 2) is needed.

> 
> If the MFN information is only available via AML, then point 2) is
> needed, although the reporting back to Xen should be restricted to a xen
> component, rather than polluting the main device driver.
> 
> 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.
>

Do you mean to treat them as mmio pages of type p2m_mmio_direct and
map them to guest by map_mmio_regions()?

Thanks,
Haozhong