Oscar Salvador <osalvador@xxxxxxx> writes: > On Mon, Mar 14, 2022 at 08:20:57AM -0700, Dave Hansen wrote: >> Qemu, for instance, has a "mem-path" argument. It's typically used for >> using hugetlbfs as guest memory. But, there's nothing stopping you from >> pointing it to a DAX device or a file on a DAX filesystem that's backed >> by pmem. > > Thanks Dave. > > But that is somehow different, is not it? > When you use pmem backed memory as a RAM for the guest, the guest is not > seeing that as PMEM, but just as a normal RAM, right? > IOW, the guest cannot use that memory for demotion, as we can use it in > the host when configured. > > I might be missing something, I am using this qemu cmdline: > > $QEMU -enable-kvm -machine pc -smp 4 -cpu host -monitor pty -m 5G \ > -object memory-backend-file,id=pc.ram,size=5G,mem-path=/mnt/pmem,share=off -machine memory-backend=pc.ram \ > $IMAGE -boot c -vnc :0 > > (/mnt/pmem was mounted with "mount -o dax /dev/pmem1 /mnt/pmem/") > > My point is, if it is really true that the guest cannot use that memory for > demotion, then we would still need CONFIG_MEMORY_HOTPLUG, as that is the > only way to expose PMEM to any system to be used as a demotion option > (via add_memory_driver_managed() through kmem driver). > > Or am I missing some qemu magic to use that memory as demotion in the > guest as well? You need to put PMEM to a NUMA node to use demotion, as follows, qemu-system-x86_64 --enable-kvm \ -M pc,accel=kvm,nvdimm=on -smp 8 -m 160G,slots=18,maxmem=703G \ -object memory-backend-ram,id=mem0,size=32G \ -object memory-backend-file,id=mem1,share=on,mem-path=/dev/dax0.0,size=128G,align=2M \ -numa node,memdev=mem0,cpus=0-7,nodeid=0 \ -numa node,memdev=mem1,nodeid=1 \ $IMAGE Best Regards, Huang, Ying
