Re: [RFC PATCH v2 0/8] ppc/spapr: VOF disk image (qcow2) boot support

[Avnish Chouhan <avnish@linux.ibm.com>]

On 2026-10-02 10:55, uverma@linux.ibm.com wrote:

Reviewed the complete series, looks good!

Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>

Regards,
Avnish Chouhan

> 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, using g_file_open_tmp() to obtain an anonymous file 
> descriptor for
>   the caller-supplied buffer. This means the ELF image has 2 copies, 
> the
>   caller's heap buffer and the temporary file. For typical PReP 
> partition sizes
>   (~4 MB) this is acceptable, but if a more memory-efficient approach 
> is
>   preferred we are open to alternatives. Feedback on the preferred 
> direction is
>   welcome.
> - 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. 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. Also opens a stdin ihandle 
> in
>   addition to stdout so GRUB can read from the console.
> 
> - FDT "disk" wildcard child node (spapr_vscsi.c): emits a single 
> generic "disk"
>   child FDT node under the vSCSI adapter (without a per-LUN suffix), 
> matching
>   the actual Open Firmware behaviour. The path_offset() wildcard 
> fallback
>   resolves any "disk@<srp-lun>" open request to this shared node; the 
> LUN is
>   extracted from the path string at runtime in vof_do_open().
> 
> 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 being discussed in the ongoing AI policy patch:
> [PATCH v4 0/4] docs/devel, AGENTS.md: a completely new policy on
> AI-generated contributions
> Link to the patch [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/20260930085433.830743-1-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             |  81 +++++-
>  hw/ppc/meson.build           |   5 +-
>  hw/ppc/spapr_vof.c           | 164 +++++++++++-
>  hw/ppc/spapr_vof_partition.c | 272 ++++++++++++++++++++
>  hw/ppc/trace-events          |   4 +
>  hw/ppc/vof.c                 | 475 ++++++++++++++++++++++++++++++++++-
>  hw/scsi/spapr_vscsi.c        |  23 ++
>  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, 1070 insertions(+), 15 deletions(-)
>  create mode 100644 hw/ppc/spapr_vof_partition.c
>  create mode 100644 include/hw/ppc/spapr_vof.h