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

["Jan Beulich" <JBeulich@suse.com>]

>>> On 13.10.16 at 10:53, <haozhong.zhang@intel.com> wrote:
> On 10/13/16 02:34 -0600, Jan Beulich wrote:
>>>>> On 12.10.16 at 18:19, <dan.j.williams@intel.com> wrote:
>>> On Wed, Oct 12, 2016 at 9:01 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>>> On 12.10.16 at 17:42, <dan.j.williams@intel.com> wrote:
>>>>> On Wed, Oct 12, 2016 at 8:39 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>>>>> On 12.10.16 at 16:58, <haozhong.zhang@intel.com> wrote:
>>>>>>> On 10/12/16 05:32 -0600, Jan Beulich wrote:
>>>>>>>>>>> On 12.10.16 at 12:33, <haozhong.zhang@intel.com> wrote:
>>>>>>>>> The layout is shown as the following diagram.
>>>>>>>>>
>>>>>>>>> +---------------+-----------+-------+----------+--------------+
>>>>>>>>> | whatever used | Partition | Super | Reserved | /dev/pmem0p1 |
>>>>>>>>> |  by kernel    |   Table   | Block | for Xen  |              |
>>>>>>>>> +---------------+-----------+-------+----------+--------------+
>>>>>>>>>                 \_____________________ _______________________/
>>>>>>>>>                                   V
>>>>>>>>>                              /dev/pmem0
>>>>>>>>
>>>>>>>>I have to admit that I dislike this, for not being OS-agnostic.
>>>>>>>>Neither should there be any Xen-specific region, nor should the
>>>>>>>>"whatever used by kernel" one be restricted to just Linux. What
>>>>>>>>I could see is an OS-reserved area ahead of the partition table,
>>>>>>>>the exact usage of which depends on which OS is currently
>>>>>>>>running (and in the Xen case this might be both Xen _and_ the
>>>>>>>>Dom0 kernel, arbitrated by a tbd protocol). After all, when
>>>>>>>>running under Xen, the Dom0 may not have a need for as much
>>>>>>>>control data as it has when running on bare hardware, for it
>>>>>>>>controlling less (if any) of the actual memory ranges when Xen
>>>>>>>>is present.
>>>>>>>>
>>>>>>>
>>>>>>> Isn't this OS-reserved area still not OS-agnostic, as it requires OS
>>>>>>> to know where the reserved area is?  Or do you mean it's not if it's
>>>>>>> defined by a protocol that is accepted by all OSes?
>>>>>>
>>>>>> The latter - we clearly won't get away without some agreement on
>>>>>> where to retrieve position and size of this area. I was simply
>>>>>> assuming that such a protocol already exists.
>>>>>>
>>>>>
>>>>> No, we should not mix the struct page reservation that the Dom0 kernel
>>>>> may actively use with the Xen reservation that the Dom0 kernel does
>>>>> not consume.  Explain again what is wrong with the partition approach?
>>>>
>>>> Not sure what was unclear in my previous reply. I don't think there
>>>> should be apriori knowledge of whether Xen is (going to be) used on
>>>> a system, and even if it gets used, but just occasionally, it would
>>>> (apart from the abstract considerations already given) be a waste
>>>> of resources to set something aside that could be used for other
>>>> purposes while Xen is not running. Static partitioning should only be
>>>> needed for persistent data.
>>>
>>> The reservation needs to be persistent / static even if the data is
>>> volatile, as is the case with struct page, because we can't have the
>>> size of the device change depending on use.  So, from the aspect of
>>> wasting space while Xen is not in use, both partitions and the
>>> intrinsic reservation approach suffer the same problem. Setting that
>>> aside I don't want to mix 2 different use cases into the same
>>> reservation.
>>
>>Then you didn't understand what I've said: I certainly didn't mean
>>the reservation to vary from a device perspective. However, when
>>Xen is in use I don't see why part of that static reservation couldn't
>>be used by Xen, and another part by the Dom0 kernel. The kernel
>>obviously would need to ask the hypervisor how much of the space
>>is left, and where that area starts.
>>
> 
> I think Dan means that there should be a clear separation between
> reservations for different usages (kernel/xen/...). The libnvdimm
> driver is for the linux kernel and only needs to maintain the
> reservation for kernel functionality. For others including xen/dm/...,
> if they want reservation for their own purpose, they should maintain
> their own reservations out of libnvdimm driver and avoid bothering the
> libnvdimm driver (e.g. add specific handling in libnvdimm driver).
> 
> IIUC, one existing example is device-mapper device (dm) which needs to
> reserve on-device area for its own meta-data. Its choice is to store
> the meta-data on the block device (/dev/pmemN) provided by the
> libnvdimm driver.
> 
> I think we can do the similar for Xen, like to lay another pseudo
> device on /dev/pmem and do the reservation, like 2. in my previous
> reply.

Well, my opinion certainly doesn't count much here, but I continue to
consider this a bad idea. For entities like drivers it may well be
appropriate, but I think there ought to be an independent concept
of "OS reserved", and in the Xen case this could then be shared
between hypervisor and Dom0 kernel. Or if we were to consider Dom0
"just a guest", things should even be the other way around: Xen gets
all of the OS reserved space, and Dom0 needs something custom.

Jan
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