Test steps:

1. Boot rhel8.5 guest with balloon device under q35, the full cmdline is as [1]. (It's not reproducible with rhel8.4 guest.)

2. Add 'page_poison=1' to guest kernel line, and reboot guest.

3. Create internal snapshot and revert it.
(qemu) savevm sn0
(qemu) loadvm sn0


Reproduce:

qemu-kvm-5.2.0-16.module+el8.4.0+12596+209e4022.10
guest kernel: 4.18.0-348.el8.x86_64
host kernel: 4.18.0-305.23.1.el8_4.x86_64

Do savevm and loadvm, hit below error.

(qemu) savevm sn2
(qemu) loadvm sn2
VQ 0 size 0x80 < last_avail_idx 0x0 - used_idx 0x44
Failed to load virtio-balloon:virtio
error while loading state for instance 0x0 of device '0000:00:05.0:00.0/virtio-balloon'
Error: Error -1 while loading VM state
(qemu) info status
VM status: paused (restore-vm)


Verify:

qemu-kvm-5.2.0-16.module+el8.4.0+13043+9eb47245.11
guest kernel: 4.18.0-348.el8.x86_64
host kernel: 4.18.0-305.23.1.el8_4.x86_64

Guest works well after savevm and loadvm, no error hit. 


[1] Full QEMU cli:
# /usr/libexec/qemu-kvm \
    -name 'avocado-vt-vm1' \
    -machine q35  \
    -nodefaults \
    -device VGA,bus=pcie.0,addr=0x1  \
    -device pvpanic,ioport=0x505,id=id5SK4co  \
    -device pcie-root-port,id=pcie.0-root-port-2,slot=2,chassis=2,addr=0x2,bus=pcie.0 \
    -device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pcie.0,addr=0x3 \
    -drive id=drive_image1,if=none,snapshot=off,aio=threads,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/rhel850-64-virtio-scsi.qcow2 \
    -device scsi-hd,id=image1,drive=drive_image1 \
    -device pcie-root-port,id=pcie.0-root-port-4,slot=4,chassis=4,addr=0x4,bus=pcie.0 \
    -device virtio-net-pci,mac=9a:39:3a:3b:3c:3d,id=idzyzw7g,vectors=4,netdev=idhia6GM,bus=pcie.0-root-port-4,addr=0x0  \
    -netdev tap,id=idhia6GM \
    -m 8192,slots=20,maxmem=80G  \
    -smp 64 \
    -cpu 'Haswell-noTSX',+kvm_pv_unhalt \
    -vnc :0  \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off,strict=off \
    -enable-kvm \
    -device pcie-root-port,id=pcie.0-root-port-5,slot=5,chassis=5,addr=0x5,bus=pcie.0 \
    -device virtio-balloon-pci,id=balloon0,bus=pcie.0-root-port-5\
    -monitor stdio \
    -serial tcp:0:1234,server,nowait \
    -monitor unix:/tmp/monitor3,server,nowait \
    -qmp tcp:0:3333,server,nowait

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: virt:av and virt-devel:av security and bug fix update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2021:4112