Hi,
Thanks to Hellbondz and dodo-dk, I succeeded in installing DSM 6.2.1 (DSM_DS3615xs_23824.pat) under proxmox 5.3.11 with Jun's Loader v1.03b.
Before last Hellbondz' post, I couldn't even boot on usb drive image.
So, if you're looking for a working vm configuration file example, you'll find below my /etc/pve/qemu-server/{VM_ID}.conf file (I just hide serial numbers and Mac address digits with *).
args: -device ich9-usb-ehci1,id=usb,multifunction=on,bus=pci.0,addr=0xa -drive file=/var/lib/vz/images/103/DS3615xs_6.2_v1.03b.img,format=raw,if=none,id=drive-usb-disk2,cache=writeback -device usb-storage,bus=usb.0,port=2,drive=drive-usb-disk2,id=usb-disk2,bootindex=1,removable=off -netdev type=tap,id=net0,ifname=tap103i0 -device e1000e,mac=**:**:**:**:**:**,netdev=net0,bus=pci.0,addr=0x12,id=net0
balloon: 512
cores: 2
memory: 1024
name: DS3615xs-621-bkp
numa: 0
ostype: l26
sata0: /dev/disk/by-id/ata-ST8000VN0022-2EL112_********,size=7814026584K
scsihw: virtio-scsi-pci
serial0: socket
smbios1: uuid=********-****-****-****-************
sockets: 1
vmgenid: ********-****-****-****-************
Concerning USB boot, for what I understood, DSM 6.2.1 does not support anymore device piix3-usb-uhci. I failed going back with it after succeeding with ich9-usb-ehci1 as shown in Hellbondz' post.
So for USB boot, you'll need this part in args parameter : (replace {VM_ID} by your proxmox virtual machine identifier or adapt boot image file path to match your own)
-device ich9-usb-ehci1,id=usb,multifunction=on,bus=pci.0,addr=0xa -drive file=/var/lib/vz/images/{VM_ID}/DS3615xs_6.2_v1.03b.img,format=raw,if=none,id=drive-usb-disk2,cache=writeback -device usb-storage,bus=usb.0,port=2,drive=drive-usb-disk2,id=usb-disk2,bootindex=1,removable=off
And for Intel E1000e network interface card, you'll need this part : (replace {VM_ID} by your proxmox virtual machine identifier and {MAC_ADDRESS} by MAC address generated by proxmox for Intel E1000 NIC)
-netdev type=tap,id=net0,ifname=tap{VM_ID}i0 -device e1000e,mac={MAC_ADDRESS},netdev=net0,bus=pci.0,addr=0x12,id=net0
For network interface, you will obtain such results with method indicated in Hellbondz' post, by executing
qm showcmd {VM_ID}
See Hellbondz post for detail, it works great!
Once again, great thanks to Hellbondz and dodo-dk work, I wouldn't have succeeded without them.
(and sorry for my bad english speaking)