Description of problem:
When set 'iommu_platform=on' on "vhost-vsock-pci" device, the following error is encountered:
(qemu) qemu-kvm: vhost_set_features failed: Operation not supported (95)
qemu-kvm: Error starting vhost: 95


Version-Release number of selected component (if applicable):
Host name: ibm-x3850x6-04.lab.eng.pek2.redhat.com
Host Distro: RHEL-8.3.0-20200912.n.0 BaseOS x86_64 
Host Kernel: 4.18.0-236.el8.x86_64
Guest Kernel: 4.18.0-236.el8.x86_64
Qemu-kvm: qemu-kvm-5.1.0-6.module+el8.3.0+8041+42ff16b8


How reproducible:
100%

Steps to Reproduce:
1. boot guest enable IOMMU:
/usr/libexec/qemu-kvm \
-name 'avocado-vt-vm1'  \
-sandbox on  \
-machine q35 \
-device pcie-root-port,id=pcie-root-port-0,multifunction=on,bus=pcie.0,addr=0x1,chassis=1 \
-device pcie-pci-bridge,id=pcie-pci-bridge-0,addr=0x0,bus=pcie-root-port-0  \
-nodefaults \
-device VGA,bus=pcie.0,addr=0x2 \
-m 30720  \
-smp 40,maxcpus=40,cores=20,threads=1,dies=1,sockets=2  \
-cpu 'IvyBridge',+kvm_pv_unhalt \
-chardev socket,server,nowait,id=qmp_id_qmpmonitor1,path=/var/tmp/monitor-qmpmonitor1  \
-mon chardev=qmp_id_qmpmonitor1,mode=control \
-device pvpanic,ioport=0x505,id=idnEJdcf \
-chardev socket,server,nowait,id=chardev_serial0,path=/var/tmp/serial-serial0 \
-device isa-serial,id=serial0,chardev=chardev_serial0  \
-chardev socket,id=seabioslog_id_20200915-055422-8y4xDLCa,path=/var/tmp/seabios,server,nowait \
-device isa-debugcon,chardev=seabioslog_id_20200915-055422-8y4xDLCa,iobase=0x402 \
-device pcie-root-port,id=pcie-root-port-1,port=0x1,addr=0x1.0x1,bus=pcie.0,chassis=2 \
-device qemu-xhci,id=usb1,bus=pcie-root-port-1,addr=0x0 \
-device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1 \
-device pcie-root-port,id=pcie-root-port-2,port=0x2,addr=0x1.0x2,bus=pcie.0,chassis=3 \
-device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pcie-root-port-2,addr=0x0 \
-blockdev node-name=file_image1,driver=file,aio=threads,filename=/home/test/os.qcow2,cache.direct=on,cache.no-flush=off \
-blockdev node-name=drive_image1,driver=qcow2,cache.direct=on,cache.no-flush=off,file=file_image1 \
-device scsi-hd,id=image1,drive=drive_image1,write-cache=on \
-device pcie-root-port,id=pcie-root-port-3,port=0x3,addr=0x1.0x3,bus=pcie.0,chassis=4 \
-device virtio-net-pci,mac=9a:af:90:80:2a:4f,id=idxXK3e9,netdev=idbkX3jT,bus=pcie-root-port-3,addr=0x0  \
-netdev tap,id=idbkX3jT,vhost=on \
-device pcie-root-port,id=pcie-root-port-4,port=0x4,addr=0x1.0x4,bus=pcie.0,chassis=5 \
-device vhost-vsock-pci,id=vhost_vsock0,guest-cid=3,bus=pcie-root-port-4,addr=0x0,iommu_platform=on  \
-vnc :20  \
-rtc base=utc,clock=host,driftfix=slew  \
-boot menu=off,order=cdn,once=c,strict=off \
-enable-kvm \
-device pcie-root-port,id=pcie_extra_root_port_0,multifunction=on,bus=pcie.0,addr=0x3,chassis=6

2. Hit this issue:
qemu-kvm: vhost_set_features failed: Operation not supported (95)
qemu-kvm: Error starting vhost: 95

3. Login guest:
[root@vm-74-49 home]# lsmod|grep vsock 
vmw_vsock_virtio_transport    16384  0
vmw_vsock_virtio_transport_common    32768  1 vmw_vsock_virtio_transport
vsock                  45056  2 vmw_vsock_virtio_transport_common,vmw_vsock_virtio_transport

[root@vm-74-49 home]# gcc -o nc-vsock nc-vsock.c
gcc: error: nc-vsock.c: No such file or directory
gcc: fatal error: no input files
compilation terminated.


Actual results:
IOMMU enable failed on "vhost-vsock-pci" device

Expected results:
IOMMU can be enabled on the "vhost-vsock-pci" device

Additional info: