When launching using an IGVM file, supply a copy of the MADT (part of the ACPI
tables) via an IGVM parameter (IGVM_VHY_MADT) to the guest, in addition to the
regular fw_cfg mechanism.

The IGVM parameter can be consumed by Coconut SVSM [1], instead of relying on
the fw_cfg interface, which has caused problems before due to unexpected access
[2,3]. Using IGVM parameters is the default way for Coconut SVSM; switching
over would allow removing specialized code paths for QEMU in Coconut.

Coconut SVSM needs to know the SMP configuration, but does not look at
any other ACPI data, nor does it interact with the PCI bus settings.
Since the MADT is static and not linked with other ACPI tables, it can
be supplied stand-alone like this.

Generating the MADT twice (IGVM processing and ACPI table building) seems
acceptable, since there is no infrastructure to obtain the MADT out of the ACPI
table memory area during IGVM processing.

In any case OVMF, which runs after SVSM has already been initialized, will
continue reading all ACPI tables via fw_cfg and provide fixed up ACPI data to
the OS as before.

This series makes ACPI table building more generic by making the BIOS linker
optional. This allows the MADT to be generated outside of the ACPI build
context. A new function (acpi_build_madt_standalone()) is added for that. With
that, the IGVM MADT parameter field can be filled with the MADT data during
processing of the IGVM file.

[1] https://github.com/coconut-svsm/svsm/pull/858
[2] https://gitlab.com/qemu-project/qemu/-/issues/2882
[3] https://github.com/coconut-svsm/svsm/issues/646

v4:
- Add ACPI table checksum calculation without BIOS linker, used
  for the standalone MADT.
- Don't pass ConfidentialGuestState into the IGVM backend anymore.
  Not needed, since we already have the full MachineState there now.
- Move the NULL check patch out into a new series (to be posted).
- Address remaining cleanup comments.

v3:
- Pass the machine state into IGVM file processing context instead of MADT data
- Generate MADT from inside the IGVM backend
- Refactor: Extract common code for finding IGVM parameter from IGVM parameter handlers
- Add NULL pointer check for igvm_get_buffer()

v2:
- Provide more context in the message of the main commit
- Document the madt parameter of IgvmCfgClass::process()
- Document why no MADT data is provided the process call in sev.c

Based-on: <20251118122133.1695767-1-kraxel@redhat.com>
Signed-off-by: Oliver Steffen <osteffen@redhat.com>

Oliver Steffen (5):
  hw/acpi: Make BIOS linker optional
  hw/acpi: Add standalone function to build MADT
  igvm: Add common function for finding parameter entries
  igvm: Pass machine state to IGVM file processing
  igvm: Fill MADT IGVM parameter field

 backends/igvm-cfg.c       |   2 +-
 backends/igvm.c           | 179 +++++++++++++++++++++++---------------
 hw/acpi/aml-build.c       |  29 +++++-
 hw/i386/acpi-build.c      |   9 ++
 hw/i386/acpi-build.h      |   2 +
 include/system/igvm-cfg.h |   3 +-
 include/system/igvm.h     |   5 +-
 target/i386/sev.c         |   3 +-
 8 files changed, 155 insertions(+), 77 deletions(-)

-- 
2.52.0