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

[Andrew Cooper]

On 11/10/16 18:51, Dan Williams wrote:
> On Tue, Oct 11, 2016 at 9:58 AM, Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx> wrote:
>> On Tue, Oct 11, 2016 at 08:53:33AM -0700, Dan Williams wrote:
>>> 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
>> Isn't it done this way with Linux? That is say if the machine has
>> 4GB of RAM and the NVDIMM is in TB range. You want to put the 'struct page'
>> for the NVDIMM ranges somewhere. That place can be in regions on the
>> NVDIMM that ndctl can reserve.
> Yes.

I do not see any sensible usecase for Xen to use NVDIMMs as plain RAM;
NVDIMMs are far more valuable for higher level management in dom0.

I certainly think that such a usecase should be out-of-scope for initial
Xen/NVDIMM support, even if only to reduce the complexity to start with.

A repeated complain I have of large feature submissions like this is
that, by trying to solve all potential usecases at one, end up being
overly complicated to develop, understand and review.

>
>>> 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.
>> This is more of just keeping track of the ranges if say the DAX file is
>> extremely fragmented and requires a lot of 'struct pages' to keep track of
>> when stiching up the VMA.
> Right, but why does the libnvdimm core need to know about this
> specific Xen reservation?  For example, if Xen wants some in-kernel
> driver to own a pmem region and place its own metadata on the device I
> would recommend something like:
>
>     bdev = blkdev_get_by_path("/dev/pmemX",  FMODE_EXCL...);
>     bdev_direct_access(bdev, ...);
>
> ...in other words, I don't think we want libnvdimm to grow new device
> types for every possible in-kernel user, Xen, MD, DM, etc. Instead,
> just claim the resulting device.

I completely agree.

Whatever ends up happening between Xen and dom0, there should be no
modifications like this to the nvdimm driver.  I will go so far as to
say that there shouldn't be any modifications to the nvdimm driver
(other than perhaps new query hooks so the Xen subsystem in Linux can
query information to then pass up to Xen, if the existing queryability
is insufficient).

~Andrew