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

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Haozhong Zhang <haozhong.zhang@intel.com>,
	linux-nvdimm@lists.01.org, xen-devel@lists.xenproject.org
Cc: Juergen Gross <jgross@suse.com>,
	Xiao Guangrong <guangrong.xiao@linux.intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	linux-kernel@vger.kernel.org,
	Stefano Stabellini <stefano@aporeto.com>,
	David Vrabel <david.vrabel@citrix.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [Xen-devel] [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for Xen
Date: Mon, 10 Oct 2016 17:43:10 +0100	[thread overview]
Message-ID: <dde78bbd-4739-98a1-4b69-2c2dff0a9d71@citrix.com> (raw)
In-Reply-To: <20161010003523.4423-1-haozhong.zhang@intel.com>

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.

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?

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.

~Andrew
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