igvm: Fill MADT IGVM parameter field on x86_64

igvm: Fill MADT IGVM parameter field on x86_64

Use the new acpi_build_madt_standalone() function to fill the MADT
parameter field.

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

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 without any changes.

The IGVM parameter handler is implemented for the i386 machine target
and stubbed for all others.

[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

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Message-ID: <20260130054714.715928-10-osteffen@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>