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

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 08/03/16 03:47, Jan Beulich wrote:
> >>> On 03.08.16 at 11:37, <haozhong.zhang@intel.com> wrote:
> > On 08/03/16 02:45, Jan Beulich wrote:
> >> >>> On 03.08.16 at 08:54, <haozhong.zhang@intel.com> wrote:
> >> > On 08/02/16 08:46, Jan Beulich wrote:
> >> >> >>> On 18.07.16 at 02:29, <haozhong.zhang@intel.com> 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).
> 

If modified in between, guest will see a virtual pmem device of
different data. But the usage of pmem is similar to disk: if a file of
the same content is given every time, the guest can get a virtual
pmem/disk of the same data as last reboot/shutdown; keeping the data
unchanged between multiple boots is out of the scope of Xen.

Haozhong

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