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

[uverma@linux.ibm.com]

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)         | 
 |                                                                            |
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 |                                                                            | 
 +----------------------------------------------------------------------------+

      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

-- 
2.54.0