Messages in this thread | | | From | Dan Williams <> | Date | Tue, 11 Oct 2016 10:51:19 -0700 | Subject | Re: [Xen-devel] [RFC KERNEL PATCH 0/2] Add Dom0 NVDIMM support for Xen |
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On Tue, Oct 11, 2016 at 9:58 AM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote: > On Tue, Oct 11, 2016 at 08:53:33AM -0700, Dan Williams wrote: >> On Tue, Oct 11, 2016 at 6:08 AM, Jan Beulich <jbeulich@suse.com> wrote: >> >>>> Andrew Cooper <andrew.cooper3@citrix.com> 10/10/16 6:44 PM >>> >> >>On 10/10/16 01:35, Haozhong Zhang wrote: >> >>> Xen hypervisor needs assistance from Dom0 Linux kernel for following tasks: >> >>> 1) Reserve an area on NVDIMM devices for Xen hypervisor to place >> >>> memory management data structures, i.e. frame table and M2P table. >> >>> 2) Report SPA ranges of NVDIMM devices and the reserved area to Xen >> >>> hypervisor. >> >> >> >>However, I can't see any justification for 1). Dom0 should not be >> >>involved in Xen's management of its own frame table and m2p. The mfns >> >>making up the pmem/pblk regions should be treated just like any other >> >>MMIO regions, and be handed wholesale to dom0 by default. >> > >> > That precludes the use as RAM extension, and I thought earlier rounds of >> > discussion had got everyone in agreement that at least for the pmem case >> > we will need some control data in Xen. >> >> The missing piece for me is why this reservation for control data >> needs to be done in the libnvdimm core? I would expect that any dax > > Isn't it done this way with Linux? That is say if the machine has > 4GB of RAM and the NVDIMM is in TB range. You want to put the 'struct page' > for the NVDIMM ranges somewhere. That place can be in regions on the > NVDIMM that ndctl can reserve.
Yes.
>> capable file could be mapped and made available to a guest. This >> includes /dev/ramX devices that are dax capable, but are external to >> the libnvdimm sub-system. > > This is more of just keeping track of the ranges if say the DAX file is > extremely fragmented and requires a lot of 'struct pages' to keep track of > when stiching up the VMA.
Right, but why does the libnvdimm core need to know about this specific Xen reservation? For example, if Xen wants some in-kernel driver to own a pmem region and place its own metadata on the device I would recommend something like:
bdev = blkdev_get_by_path("/dev/pmemX", FMODE_EXCL...); bdev_direct_access(bdev, ...);
...in other words, I don't think we want libnvdimm to grow new device types for every possible in-kernel user, Xen, MD, DM, etc. Instead, just claim the resulting device.
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