Re: [Xen-devel] [RFC Design Doc v2] Add vNVDIMM support for Xen

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.08.16 at 11:37, <haozhong.zhang@xxxxxxxxx> wrote:
> On 08/03/16 02:45, Jan Beulich wrote:
>> >>> On 03.08.16 at 08:54, <haozhong.zhang@xxxxxxxxx> wrote:
>> > On 08/02/16 08:46, Jan Beulich wrote:
>> >> >>> On 18.07.16 at 02:29, <haozhong.zhang@xxxxxxxxx> wrote:
>> >> >  (4) Because the reserved area is now used by Xen hypervisor, it
>> >> >      should not be accessible by Dom0 any more. Therefore, if a host
>> >> >      pmem device is recorded by Xen hypervisor, Xen will unmap its
>> >> >      reserved area from Dom0. Our design also needs to extend Linux
>> >> >      NVDIMM driver to "balloon out" the reserved area after it
>> >> >      successfully reports a pmem device to Xen hypervisor.
>> >> 
>> >> ... "balloon out" ... _after_? That'd be unsafe.
>> >>
>> > 
>> > Before ballooning is accomplished, the pmem driver does not create any
>> > device node under /dev/ and hence no one except the pmem drive can
>> > access the reserved area on pmem, so I think it's okey to balloon
>> > after reporting.
>> 
>> Right now Dom0 isn't allowed to access any memory in use by Xen
>> (and not explicitly shared), and I don't think we should deviate
>> from that model for pmem.
> 
> In this design, Xen hypervisor unmaps the reserved area from Dom0 so
> that Dom0 cannot access the reserved area afterwards. And "balloon" is
> in fact not a memory ballooning, because Linux kernel never allocates
> from pmem like normal ram. In my current implementation, it's just to
> remove the reserved area from a resource struct covering pmem.

Ah, in that case please either use a different term, or explain what
"balloon out" is meant to mean in this context.

>> >> > 4.2.3 Get Host Machine Address (SPA) of Host pmem Files
>> >> > 
>> >> >  Before a pmem file is assigned to a domain, we need to know the host
>> >> >  SPA ranges that are allocated to this file. We do this work in xl.
>> >> > 
>> >> >  If a pmem device /dev/pmem0 is given, xl will read
>> >> >  /sys/block/pmem0/device/{resource,size} respectively for the start
>> >> >  SPA and size of the pmem device.
>> >> > 
>> >> >  If a pre-allocated file /mnt/dax/file is given,
>> >> >  (1) xl first finds the host pmem device where /mnt/dax/file is. Then
>> >> >      it uses the method above to get the start SPA of the host pmem
>> >> >      device.
>> >> >  (2) xl then uses fiemap ioctl to get the extend mappings of
>> >> >      /mnt/dax/file, and adds the corresponding physical offsets and
>> >> >      lengths in each mapping entries to above start SPA to get the SPA
>> >> >      ranges pre-allocated for this file.
>> >> 
>> >> Remind me again: These extents never change, not even across
>> >> reboot? I think this would be good to be written down here explicitly.
>> > 
>> > Yes
>> > 
>> >> Hadn't there been talk of using labels to be able to allow a guest to
>> >> own the exact same physical range again after reboot or guest or
>> >> host?
>> > 
>> > You mean labels in NVDIMM label storage area? As defined in Intel
>> > NVDIMM Namespace Specification, labels are used to specify
>> > namespaces. For a pmem interleave set (possible cross several dimms),
>> > at most one pmem namespace (and hence at most one label) is
>> > allowed. Therefore, labels can not be used to partition pmem.
>> 
>> Okay. But then how do particular ranges get associated with the
>> owning guest(s)? Merely by SPA would seem rather fragile to me.
>> 
> 
> By using the file name, e.g. if I specify vnvdimm = [ 'file=/mnt/dax/foo' ]
> in a domain config file, SPA occupied by /mnt/dax/foo are mapped to
> the domain.  If the same file is used every time the domain is created,
> the same virtual device will be seen by that domain.

So what if the file got deleted and re-created in between? Since
I don't think you can specify the SPAs to use when creating such
a file, such an operation would be quite different from removing
and re-adding e.g. a specific PCI device (to be used by a guest)
on a host (while the guest is not running).

Jan


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