Comment 19 for bug 1848790 - I tested the compiled kernel on my RaspberryPi4 (4GB RAM model). Both KVM and external SSD/USB (in my case Samsung T5, 500GB external SSD) on the USB 3.0 port are working fine:

root@cloudstack-mgmt:~# ls /dev/kvm
root@cloudstack-mgmt:~# kvm-ok
INFO: /dev/kvm exists
KVM acceleration can be used

root@cloudstack-mgmt:~# lsusb
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 002: ID 04e8:61f5 Samsung Electronics Co., Ltd Portable SSD T5
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 002: ID 2109:3431 VIA Labs, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

# fdisk -l
[.. snipped ..]
Disk /dev/sda: 465.78 GiB, 500107862016 bytes, 976773168 sectors
Disk model: Portable SSD T5
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device Boot Start End Sectors Size Id Type
/dev/sda1 2048 976773134 976771087 465.8G c W95 FAT32 (LBA)

root@cloudstack-mgmt:~# blkid -o list
device fs_type label mount point UUID
/dev/mmcblk0p1 vfat system-boot /boot/firmware 79A2-EC62
/dev/mmcblk0p2 ext4 writable / 2fae1f20-4a20-4c77-a39b-fb6480f64a0b
/dev/loop0 squashfs /snap/lxd/12210
/dev/loop1 squashfs /snap/core/7920
/dev/sda1 ext4 (not mounted) 91175b3a-ee2c-47a7-a1e5-f4528e127523

This confirms that the patch and kernel-image build by fixes the external USB storage issue with the 4GB RPi4 model. Thanks