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

[Dan Williams]

On Tue, Oct 11, 2016 at 6:08 AM, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 10/10/16 6:44 PM >>>
>>On 10/10/16 01:35, Haozhong Zhang wrote:
>>> 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.
>>
>>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.
>
> That precludes the use as RAM extension, and I thought earlier rounds of
> discussion had got everyone in agreement that at least for the pmem case
> we will need some control data in Xen.

The missing piece for me is why this reservation for control data
needs to be done in the libnvdimm core?  I would expect that any dax
capable file could be mapped and made available to a guest.  This
includes /dev/ramX devices that are dax capable, but are external to
the libnvdimm sub-system.