[SeaBIOS] qemu / seabios ACPI table interface

Laszlo Ersek lersek at redhat.com
Fri Mar 22 17:09:53 CET 2013

I'm confused. What are the requirements?

(1) should unpatched qemu work with patched seabios?
(2) should patched qemu work with unpatched seabios?

Considering patched qemu + patched seabios,
(3) should qemu dynamically control table origin/contents per table?
(4) should qemu be able to suppress/disable a seabios table via fw_cfg
without providing a replacement?

I had thought:
(1) yes (firmware upgrade on same hardware),
(2) no (hardware upgrade),
(3) yes (eases development and ultimately covers everything),
(4) no (*)

but apparently I'm wrong. I'm ready to be enlightened and try to
implement whatever the consensus is, but what is the consensus?

(*) For each table we can investigate why SeaBIOS provides it now:

- RSDP, RSDT, XSDT: These look like links-only tables. In general, links
(pointers) have to be updated by the firmware (eg. in the FADT), thus
qemu would provide zeros in those fields in general. Since these three
tables consist of nothing more than pointers, qemu would never provide
any of these tables. Choice between RSDT and XSDT is the jurisdiction of
the boot firmware, and it should choose exactly one. (SeaBIOS currently
uses RSDT, OVMF uses XSDT.)

- FADT (FACP): required by the spec. Qemu would either put up with
SeaBIOS's or provide a replacement.

- FACS: always prepared by the boot firmware.

- DSDT: Same as FADT.

- SSDT: a continuation of the DSDT; there can be several instances. If
the DSDT comes from qemu, SeaBIOS shouldn't install any SSDTs of its
own. Qemu won't provide SSDTs without its own DSDT.

- APIC (MADT): required in the "APIC interrupt model", which is probably
"always" for us. Hence same as with FADT/DSDT.

- HPET: Hardware dependent. If qemu doesn't provide the hardware, it
also wouldn't provide the table, and SeaBIOS already doesn't install one
itself because there's no hardware. If the hardware is there, qemu
provides the table, or puts up with SeaBIOS's.

- SRAT: Dependent on NUMA setup. Same case as with HPET.

- MCFG: Seems to depend on hardware (q35). Same as with HPET.

(I'll be out of the office next week -- if I don't follow up, that's the


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