Re: [RFC PATCH 0/8] ppc/spapr: VOF disk image (qcow2) boot support

[Utkarsh Verma <uverma@linux.ibm.com>]

+ Alexey (AMD)

Hi Alexey,
Will you continue to maintain VOF in QEMU?
Could you please share your feedback on this patch series?

Regards,
Utkarsh

On 8/17/26 3:57 PM, uverma@linux.ibm.com wrote:
> From: Utkarsh Verma <uverma@linux.ibm.com>
> 
> This series adds support for booting from a disk image under the Virtual Open
> Firmware (VOF) path for sPAPR/pseries machines in QEMU.
> 
> The most common use case for pseries VMs is booting from a distro qcow2 disk
> image. These images carry a PReP boot partition containing GRUB2, which in turn
> loads the kernel and initrd via IEEE 1275 client interface calls.
> VOF currently only supports direct -kernel/-initrd boot; there is no
> firmware-mediated path to hand off to a bootloader on disk. This series
> implements that missing path, making VOF capable of handling the standard
> distro qcow2 boot flow without requiring SLOF.
> 
> This series takes reference from an earlier implementation by Alexey
> Kardashevskiy [1] which explored the same concept but was not carried forward at
> the time.
> 
> The existing load_elf function works on a file, whereas in the VOF the PReP
> partition (GRUB) is read from the disk image and loaded in the memory, also
> on loading the PReP partition in the guest memory the /memory@0/available FDT
> needs to be properly updated for each PT_LOAD segment of the core.elf file
> of the GRUB (PReP partition).
> 
> Implemented a new public elf loader API load_elf_ram_sym_buf() to resolve both
> these issues. load_elf_ram_sym_buf() accepts a caller-supplied memory buffer
> instead of a filename. Internally it writes the buffer to an anonymous memfd
> and delegates to the existing load_elf32/load_elf64 paths, so all existing
> behaviour is preserved. Also, added optional segment_fn_t callback in
> load_elf_ram_sym_buf() through the internal elf_ops.h.inc template; it is
> invoked once per PT_LOAD segment after the load address is resolved, allowing
> callers to perform per-segment work such as VOF memory claims.
> 
> When no -kernel is given, spapr_vof_reset() calls a newly implemented function
> spapr_vof_try_prep_boot() which iterates attached block backends, scans
> partitions for a PReP boot partition, reads the partition contents into a
> host buffer, calls load_elf_ram_sym_buf() to load the GRUB in the guest memory,
> and on success stores the ELF entry point in spapr->kernel_addr.
> spapr_vof_client_dt_finalize() then writes the entry point into
> /chosen/qemu,boot-kernel so VOF then transfers control to GRUB.
> 
> - Partition scanner (spapr_vof_partition.c): scans MBR and GPT partition tables
>    on a BlockBackend to locate the PReP boot partition.
> 
> - Core loader extension (hw/core/loader.c): new load_elf_ram_sym_buf() API loads
>    a 32/64-bit ELF binary from a caller-supplied memory buffer, reusing the
>    existing load_elf32/load_elf64 paths via a memfd. A new segment_fn_t callback
>    in elf_ops.h.inc lets callers act on each PT_LOAD segment as it is resolved.
> 
> - read, seek OF services: two previously unimplemented IEEE 1275 client
>    services, covering block device I/O needed by GRUB.
> 
> - vscsi-report-luns method: walks the SCSIBus and builds the LUN report table
>    in guest memory in the format expected by GRUB's ofdisk driver, matching
>    SLOF's dev-generate-srplun encoding.
> 
> - VTY write/read routing: routes console I/O through the VTY chardev backend;
>    promotes vty_getchars to external linkage.
> 
> - FDT disk@<lun> child nodes (spapr_vscsi.c): emits a child FDT node per
>    attached SCSI disk in VOF mode, giving GRUB a fully-qualified OF path to open.
>    Gated on spapr->vof so SLOF boots are unaffected.
> 
> We used AI to build prototype with manual changes incorporated as identified
> during debugging multiple issues faced while making it work, hence using below
> tag as suggested in [2].
> AI-used-for: Code
> 
> Testing:
> 
> The series has been tested end-to-end by booting a RHEL9.7 qcow2 image on a
> pseries machine with VOF enabled (no SLOF).
> 
> build$ ./qemu-system-ppc64 -M pseries,x-vof=on -cpu power9 -m 2G \
> -hda ../../rhel-guest-image-9.7-20251021.0.ppc64le.qcow2 -nographic
> qemu-system-ppc64: warning: TCG doesn't support requested feature, cap-cfpc=workaround
> qemu-system-ppc64: warning: TCG doesn't support requested feature, cap-ibs=workaround
> qemu-system-ppc64: warning: TCG doesn't support requested feature, cap-ccf-assist=on
> Welcome to GRUB!
> <snip>
>                                 GRUB version 2.06
> 
>   +----------------------------------------------------------------------------+
>   |*Red Hat Enterprise Linux (5.14.0-611.5.1.el9_7.ppc64le) 9.7 (Plow)         |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   |                                                                            |
>   +----------------------------------------------------------------------------+
> 
>        Use the ^ and v keys to select which entry is highlighted.
>        Press enter to boot the selected OS, `e' to edit the commands
>        before booting or `c' for a command-line.
> 
> <snip>
> 
> Red Hat Enterprise Linux 9.7 (Plow)
> Kernel 5.14.0-611.5.1.el9_7.ppc64le on an ppc64le
> 
> Activate the web console with: systemctl enable --now cockpit.socket
> 
> localhost login: root
> Password:
> Last login: Fri Jul 10 01:45:44 on hvc0
> [root@localhost ~]#
> [root@localhost ~]# ls /
> afs  boot  etc	 lib	media  opt   root  sbin  sys  usr
> bin  dev   home  lib64	mnt    proc  run   srv	 tmp  var
> [root@localhost ~]#
> [root@localhost ~]# echo "Hello, World!" > hello.txt
> [root@localhost ~]# cat hello.txt
> Hello, World!
> [root@localhost ~]#
> 
> [1] https://lore.kernel.org/qemu-devel/20200203032943.121178-1-aik@ozlabs.ru/#t
> [2] https://lore.kernel.org/qemu-devel/20260529094619.1034458-1-pbonzini@redhat.com/
> 
> Utkarsh Verma (8):
>    ppc/spapr: add PReP boot partition detection
>    hw/loader: add load_elf_ram_sym_buf() for in-memory ELF loading
>    ppc/spapr: add baseline VOF disk boot support
>    ppc/spapr: add VTY backend support to OF read/write/open services
>    ppc/spapr: add block device backend to VOF open/read/write/seek
>      services
>    spapr_vscsi: add VOF disk nodes to the device tree
>    ppc/spapr: strip OF path argument suffix in path_offset
>    ppc/spapr: implement vscsi-report-luns call-method for PAPR vSCSI
> 
>   hw/char/spapr_vty.c          |   2 +-
>   hw/core/loader.c             |  77 ++++++-
>   hw/ppc/meson.build           |   5 +-
>   hw/ppc/spapr_vof.c           | 144 +++++++++++-
>   hw/ppc/spapr_vof_partition.c | 176 ++++++++++++++
>   hw/ppc/trace-events          |   4 +
>   hw/ppc/vof.c                 | 429 +++++++++++++++++++++++++++++++++++
>   hw/scsi/spapr_vscsi.c        |  40 ++++
>   include/hw/core/loader.h     |  30 +++
>   include/hw/elf_ops.h.inc     |  12 +-
>   include/hw/ppc/spapr_vio.h   |   1 +
>   include/hw/ppc/spapr_vof.h   |  14 ++
>   include/hw/ppc/vof.h         |   2 +
>   13 files changed, 923 insertions(+), 13 deletions(-)
>   create mode 100644 hw/ppc/spapr_vof_partition.c
>   create mode 100644 include/hw/ppc/spapr_vof.h
>