This fixes the bug in OVMF's Qemu ACPI table linker which caused it to
fail or misbehave when multiple fields point to the same table.
Previously, each pointer caused the pointed-to table to be installed
via the EFI_ACPI_TABLE_PROTOCOL. However, each table must only be
installed once via this mechanism.

The patch fixes this (as previously suggested by Laszlo) via memoisation
of the pointers. If it encounters the same pointer twice, it will no
longer try to process it again.

I've tested it successfully with Windows 10, a recent Ubuntu
version, and OS X as guest OSes. (further OVMF patches required for
booting the latter)

I found this bug while trying to patch Qemu to generate a Rev3 FADT (as
per ACPI 2.0) instead of a Rev1 FADT (ACPI 1.0). Said patch has missed
the Qemu 2.9 merge window, but has been provisionally accepted for 2.10
(save for some minor tweaks) and will break the ACPI table linker
on unpatched OVMF. (When this bug is triggered, OVMF reverts to its
built-in ACPI tables, ignoring those provided by Qemu.)

v1 -> v2
- The only functional change is that the memoisation happens earlier
  in the processing of QEMU_LOADER_ADD_POINTER commands; right at the
  start, not immediately before installing the table. [Laszlo]
- Other changes are documentation/naming/style. (see patch 1/1) [Laszlo]

Previous discussion: https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg06679.html

Qemu patch: https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg02837.html

Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=368

Github feature branch: https://github.com/pmj/edk2/tree/bug_368_v2

Phil Dennis-Jordan (1):
  OvmfPkg: Allow multiple add-pointer linker commands to same ACPI table

 OvmfPkg/AcpiPlatformDxe/QemuFwCfgAcpi.c | 109 ++++++++++++++++++--
 1 file changed, 98 insertions(+), 11 deletions(-)

-- 
2.3.2 (Apple Git-55)

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