What were you trying to do that didn't work?

Use a virtiofs filesystem on an SE guest with memory backing source a file.

What is the impact of this issue to you?

VMs must be defined to use memory backing with memfd.

Please provide the package NVR for which the bug is seen:

host
kernel-6.11.0-0.rc5.22.el10.s390x
qemu-kvm-9.1.0-1.el10.s390x
libvirt-10.5.0-5.el10.s390x

guest
kernel-6.11.0-0.rc5.22.el10.s390x

How reproducible is this bug?:

100%

Steps to reproduce

1. Set memory backing

   <memoryBacking><source type="file"/><access mode="shared"/></memoryBacking>

   on a VM with

   <launchSecurity type="s390-pv"/>

   The VM has sufficient memory of 2GB.

1. Start VM

Expected results

The VM boots correctly. I can log in.

Actual results

The boot sequence gets stuck very early:

Escape character is ^] (Ctrl + ])
LOADPARM=[        ]
Using virtio-blk.
Using SCSI scheme.
.........................................................................................................................

Additional notes

1. When SE was introduced virtiofs was not supported. At some point we got it from upstream. For SE we only run a critical subset of known test cases. Therefore we only covered //source@type=file in the past.
2. There are three memory backing types: memfd, file, hugepages. On 9.5 memfd and file worked, hugepages never worked, but there's an additional line in the guest output:

   Escape character is ^] (Ctrl + ])
   LOADPARM=[        ]
   Using virtio-blk.
   Using SCSI scheme.
   .........................................................................................................................
   Protected boot has failed: 0xa02

1. From the following two test cases, only the 'memfd_backed' now passes on 10.0.beta:

   JOB ID     : ffab891ab8f7015665bc7c0a848332fffeaf90e2
   JOB LOG    : /var/log/avocado/job-results/job-2024-09-10T09.17-ffab891/job.log
    (1/2) type_specific.io-github-autotest-libvirt.virtual_devices.filesystem_device.positive_test.file_backed.nop.fs_test.xattr_on.cache_mode_auto.thread_pool_noset.one_fs.one_guest: PASS (26.22 s)
    (2/2) type_specific.io-github-autotest-libvirt.virtual_devices.filesystem_device.positive_test.memfd_backed.nop.fs_test.xattr_on.cache_mode_auto.thread_pool_noset.one_fs.one_guest: PASS (24.33 s)
   RESULTS    : PASS 2 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
   JOB HTML   : /var/log/avocado/job-results/job-2024-09-10T09.17-ffab891/results.html
   JOB TIME   : 50.98 s
   

2. I noticed when the VM is stuck as described with type=file, I can't even take a memory dump (virsh dump --memory-only).
3. Libvirt starts the file backed VM with qemu cmdline

   -machine s390-ccw-virtio-rhel9.4.0,usb=off,dump-guest-core=off,memory-backend=s390.ram,confidential-guest-support=lsec0  -object {"qom-type":"memory-backend-file","id":"s390.ram","mem-path":"/var/lib/libvirt/qemu/ram/1-avocado-vt-vm1/s390.ram","share":true,"x-use-canonical-path-for-ramblock-id":false,"size":1073741824} 

   .