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

["Zhang, Haozhong" <haozhong.zhang@xxxxxxxxx>]

Hi Kevin,

Thanks for your review!

On 02/02/16 14:33, Tian, Kevin wrote:
> > From: Zhang, Haozhong
> > Sent: Monday, February 01, 2016 1:44 PM
> > 
> [...]
> > 
> > 1.2 ACPI Support
> > 
> >  ACPI provides two factors of support for NVDIMM. First, NVDIMM
> >  devices are described by firmware (BIOS/EFI) to OS via ACPI-defined
> >  NVDIMM Firmware Interface Table (NFIT). Second, several functions of
> >  NVDIMM, including operations on namespace labels, S.M.A.R.T and
> >  hotplug, are provided by ACPI methods (_DSM and _FIT).
> > 
> > 1.2.1 NFIT
> > 
> >  NFIT is a new system description table added in ACPI v6 with
> >  signature "NFIT". It contains a set of structures.
> 
> Can I consider only NFIT as a minimal requirement, while other stuff
> (_DSM and _FIT) are optional?
>

No. ACPI namespace devices for NVDIMM should also be present. However,
_DSM under those ACPI namespace device can be implemented to support
no functions. _FIT is optional and is used for NVDIMM hotplug.

> > 
> > 
> > 2. NVDIMM/vNVDIMM Support in Linux Kernel/KVM/QEMU
> > 
> > 2.1 NVDIMM Driver in Linux Kernel
> > 
> [...]
> > 
> >  Userspace applications can mmap(2) the whole pmem into its own
> >  virtual address space. Linux kernel maps the system physical address
> >  space range occupied by pmem into the virtual address space, so that every
> >  normal memory loads/writes with proper flushing instructions are
> >  applied to the underlying pmem NVDIMM regions.
> > 
> >  Alternatively, a DAX file system can be made on /dev/pmemX. Files on
> >  that file system can be used in the same way as above. As Linux
> >  kernel maps the system address space range occupied by those files on
> >  NVDIMM to the virtual address space, reads/writes on those files are
> >  applied to the underlying NVDIMM regions as well.
> 
> Does it mean only file-based interface is supported by Linux today, and 
> pmem aware application cannot use normal memory allocation interface
> like malloc for the purpose?
>

right

> > 
> > 2.2 vNVDIMM Implementation in KVM/QEMU
> > 
> >  (1) Address Mapping
> > 
> >   As described before, the host Linux NVDIMM driver provides a block
> >   device interface (/dev/pmem0 at the bottom) for a pmem NVDIMM
> >   region. QEMU can than mmap(2) that device into its virtual address
> >   space (buf). QEMU is responsible to find a proper guest physical
> >   address space range that is large enough to hold /dev/pmem0. Then
> >   QEMU passes the virtual address of mmapped buf to a KVM API
> >   KVM_SET_USER_MEMORY_REGION that maps in EPT the host physical
> >   address range of buf to the guest physical address space range where
> >   the virtual pmem device will be.
> > 
> >   In this way, all guest writes/reads on the virtual pmem device is
> >   applied directly to the host one.
> > 
> >   Besides, above implementation also allows to back a virtual pmem
> >   device by a mmapped regular file or a piece of ordinary ram.
> 
> What's the point of backing pmem with ordinary ram? I can buy-in
> the value of file-backed option which although slower does sustain
> the persistency attribute. However with ram-backed method there's
> no persistency so violates guest expectation.
>

Well, it is not a necessity. The current vNVDIMM implementation in
QEMU uses dimm in QEMU that happens to support ram backend. A possible
usage is for debugging vNVDIMM on machines without NVDIMM.

> btw, how is persistency guaranteed in KVM/QEMU, cross guest 
> power off/on? I guess since Qemu process is killed the allocated pmem
> will be freed so you may switch to file-backed method to keep 
> persistency (however copy would take time for large pmem trunk). Or
> will you find some way to keep pmem managed separated from qemu
> qemu life-cycle (then pmem is not efficiently reused)?
>

It all depends on guests themselves. clwb/clflushopt/pcommit
instructions are exposed to guest that are used by guests to make
writes to pmem persistent.

Haozhong

> > 3. Design of vNVDIMM in Xen
> > 
> > 3.2 Address Mapping
> > 
> > 3.2.2 Alternative Design
> > 
> >  Jan Beulich's comments [7] on my question "why must pmem resource
> >  management and partition be done in hypervisor":
> >  | Because that's where memory management belongs. And PMEM,
> >  | other than PBLK, is just another form of RAM.
> >  | ...
> >  | The main issue is that this would imo be a layering violation
> > 
> >  George Dunlap's comments [8]:
> >  | This is not the case for PMEM.  The whole point of PMEM (correct me if
> >    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ used as fungible ram
> >  | I'm wrong) is to be used for long-term storage that survives over
> >  | reboot.  It matters very much that a guest be given the same PRAM
> >  | after the host is rebooted that it was given before.  It doesn't make
> >  | any sense to manage it the way Xen currently manages RAM (i.e., that
> >  | you request a page and get whatever Xen happens to give you).
> >  |
> >  | So if Xen is going to use PMEM, it will have to invent an entirely new
> >  | interface for guests, and it will have to keep track of those
> >  | resources across host reboots.  In other words, it will have to
> >  | duplicate all the work that Linux already does.  What do we gain from
> >  | that duplication?  Why not just leverage what's already implemented in
> >  | dom0?
> >  and [9]:
> >  | Oh, right -- yes, if the usage model of PRAM is just "cheap slow RAM",
> >  | then you're right -- it is just another form of RAM, that should be
> >  | treated no differently than say, lowmem: a fungible resource that can be
> >  | requested by setting a flag.
> > 
> >  However, pmem is used more as persistent storage than fungible ram,
> >  and my design is for the former usage. I would like to leave the
> >  detection, driver and partition (either through namespace or file
> >  systems) of NVDIMM in Dom0 Linux kernel.
> 
> After reading the whole introduction I vote for this option too. One immediate
> reason why a resource should be managed in Xen, is whether Xen itself also
> uses it, e.g. normal RAM. In that case Xen has to control the whole resource 
> to 
> protect itself from Dom0 and other user VMs. Given a resource not used by
> Xen completely, it's reasonable to leave it to Dom0 which reduces code 
> duplication
> and unnecessary maintenance burden in Xen side, as we have done for whole
> PCI sub-system and other I/O peripherals. I'm not sure whether there's future
> value to use pmem in Xen itself, at least for now the primary requirement is 
> about exposing pmem to guest. From that angle reusing NVDIMM driver in
> Dom0 looks the better choice with less enabling effort to catch up with KVM.
> 
> Thanks
> Kevin

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