From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Zhang, Haozhong" Subject: Re: [RFC Design Doc] Add vNVDIMM support for Xen Date: Tue, 2 Feb 2016 15:39:02 +0800 Message-ID: <20160202073901.GI6293@hz-desktop.sh.intel.com> References: <20160201054414.GA25211@hz-desktop.sh.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: "Tian, Kevin" Cc: Juergen Gross , Wei Liu , Ian Campbell , George Dunlap , Andrew Cooper , Stefano Stabellini , Ian Jackson , "xen-devel@lists.xen.org" , Jan Beulich , "Nakajima, Jun" , Xiao Guangrong , Keir Fraser List-Id: xen-devel@lists.xenproject.org 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