[PATCH 1/2] i386/acpi: fix inconsistent QEMU/OVMF device paths

["Michael S. Tsirkin" <mst@redhat.com>]

macOS uses ACPI UIDs to build the DevicePath for NVRAM boot options,
while OVMF firmware gets them via an internal channel through QEMU.
Due to a bug in QEMU ACPI currently UEFI firmware and ACPI have
different values, and this makes the underlying operating system
unable to report its boot option.

The particular node in question is the primary PciRoot (PCI0 in ACPI),
which for some reason gets assigned 1 in ACPI UID and 0 in the
DevicePath. This is due to the _UID assigned to it by build_dsdt in
hw/i386/acpi-build.c Which does not correspond to the primary PCI
identifier given by pcibus_num in hw/pci/pci.c

Reference with the device paths, OVMF startup logs, and ACPI table
dumps (SysReport):
https://github.com/acidanthera/bugtracker/issues/1050

In UEFI v2.8, section "10.4.2 Rules with ACPI _HID and _UID" ends with
the paragraph,

    Root PCI bridges will use the plug and play ID of PNP0A03, This will
    be stored in the ACPI Device Path _HID field, or in the Expanded
    ACPI Device Path _CID field to match the ACPI name space. The _UID
    in the ACPI Device Path structure must match the _UID in the ACPI
    name space.

(See especially the last sentence.)

Considering *extra* root bridges / root buses (with bus number > 0),
QEMU's ACPI generator actually does the right thing; since QEMU commit
c96d9286a6d7 ("i386/acpi-build: more traditional _UID and _HID for PXB
root buses", 2015-06-11).

However, the _UID values for root bridge zero (on both i440fx and q35)
have always been "wrong" (from UEFI perspective), going back in QEMU to
commit 74523b850189 ("i386: add ACPI table files from seabios",
2013-10-14).

Even in SeaBIOS, these _UID values have always been 1; see commit
a4d357638c57 ("Port rombios32 code from bochs-bios.", 2008-03-08) for
i440fx, and commit ecbe3fd61511 ("seabios: q35: add dsdt", 2012-12-01)
for q35.

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: vit9696 <vit9696@protonmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/i386/acpi-build.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index b7bcbbbb2a..7a5a8b3521 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -1497,7 +1497,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
         dev = aml_device("PCI0");
         aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0A03")));
         aml_append(dev, aml_name_decl("_ADR", aml_int(0)));
-        aml_append(dev, aml_name_decl("_UID", aml_int(1)));
+        aml_append(dev, aml_name_decl("_UID", aml_int(0)));
         aml_append(sb_scope, dev);
         aml_append(dsdt, sb_scope);
 
@@ -1512,7 +1512,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
         aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0A08")));
         aml_append(dev, aml_name_decl("_CID", aml_eisaid("PNP0A03")));
         aml_append(dev, aml_name_decl("_ADR", aml_int(0)));
-        aml_append(dev, aml_name_decl("_UID", aml_int(1)));
+        aml_append(dev, aml_name_decl("_UID", aml_int(0)));
         aml_append(dev, build_q35_osc_method());
         aml_append(sb_scope, dev);
         aml_append(dsdt, sb_scope);
-- 
MST